I have recently acquired a RDLC 78 cc two stroke which started life in Germany in 1986. I am told these are now quite rare. Can anyone tell me how many are now registered with DVLA.

no engine told at all and is a 100% engine EFI questions.
NO VIN told and I tell you what is under the hood,
no country told but VIN covers that too.
sadly there is only generic answers.
guess #1 gasoline , not diesel.
engines die for over 50 causes,. we do tests to find them.
here is list of many, fuel pump is only 1.
no tuneups done for 18 years, a hopeless bit of abuse.???
spark parts, filters all, belts. fluids, etc.
cam belt slipped, never serviced for 60 to 100k miles BOOM bad
all 3 engines used in USA version have this belt, that must never be ignored, other pistons bend the heave valves over. horror.
bad spark, the ECU sees spark dead it cuts fuel, fuel pump be danged.
and last is fueling, never first , above is first
what I do is check cam timing first not make it worse cranking.
.

world wide list,
1,984 cc (2.0 L) B5204T5 turbo I5
2,319 cc (2.3 L) B5234T3 turbo I5
2,435 cc (2.4 L) B5244S/S2 I5
2,435 cc (2.4 L) B5244SG I5 (petrol / LPG / CNG)
2,435 cc (2.4 L) B5244T/T3/T5 turbo I5
2,521 cc (2.5 L) B5254T2/T2-R turbo I5
2,435 cc (2.4 L) D5244T turbodiesel I5

https://www.fleetwoodrv.com/owners-manuals/

Change the ignition switch, and look at the car racks, only here the chance that you will find it is very small! Because your model is quite rare for the USA, or you can find it on some forums / Vw communities, enthusiasts may well know! VW club

ENGINE 2 stroke single cylinder direct reed valve crankcase induction Liquid cooled 125 cc engine Cylinder size 124 8 cc Bore and stroke 54 x 54 5 mm 200 cc engine Cylinder size 175 3 cc Bore and stroke 64 x 54 5 mm 250 cc engine Cylinder size 247 7 cc Bore and stroke 72 5 x 60 mm 280 cc engine Cylinder size 272 2 cc Bore and stroke 76 x 60 mm 300 cc engine Cylinder size 294 1 cc Bore and stroke 79 x 60 mm I Cant put periods or commas so bare with me

Why not make a manifold & go with fuel injection?
When you say webers are pricy, your working on a
rare project
Those usually don't have a budget & the people have deep
pockets that play with those hobbies

check or replace battery terminals, I replaced my terminals

check your coil it could be shorting out

My instrument panel is doing the exact same thing. It started today. I almost think it may be a loose cable to the computer that drives the lights. Did you ever resolve your issue?

ecm and bcm located in trunk but what is happening will probably not show up sounds like a torque converter lock up whicxh would explain reving up and down switching out filters in a 1986 tranny need to be carful especially concidering that porbley was never down make sur eyou flushed out completely several time because if u did not you'll only get a couoke of weeks out of that tranny before it dies flushing older car trannys is dangerouse bcause of years of sludge build up around gears and vital parts flushiung will leave behind these debris and when the new fluid filters through syatem it will break up causing huge probems so keep that in mind but this could be a simple sensor issue

well you didn't specify the year, make, and model of the truck you are asking about so this is the best answer I can come up with. Below is a list I found that says what vehicles are interference motors...

INTERFERENCE ENGINES WITH TIMING BELTS
ACURA
1986-89 1.6L Integra
1991-95 1.7L Integra
1990-95 1.8L Integra
1986-89 2.5L Legend
1992-94 2.5L Vigor
1986-89 2.7L Legend
1990 2.7L Legend
1991-95 3.0L NSX
1991-95 3.2L Legend
AUDI
1970-93 All Except 1970-77
1.9L & 1970-73 1.8L
BMW
1987-95 2.5L 325I 525I
1994-95 4.0L 740I
CHRYSLER
1993-95 1.5L Colt
1987-88 1.5L Colt
1992-95 1.5L Eagle Summit
1987-88 1.6L Colt
1989-92 1.6L Eagle Summit
1994-98 2.0L Neon Stratus
1990-95 2.0L Eagle Talon
DAIHATSU
1988-92 1.0L Charade
1988-92 1.3L Charade
1990-92 1.6L Rocky
FIAT
1974-79 1.3L 128 Series
1979-82 1.5L Stranda
1974-78 1.6L 124 Series
1974-78 1.8L 124 Series
1974-78 1.8L 131 Series, Brava
1979-82 2.0L Brava, Spider
FORD
1981-85 1.6L Escort, EXP
1981-83 1.6L LN7, Lynx
1984-85 2.0L Escort, Tempo
1993-95 2.0L Probe
1986-88 2.0L Ranger
1984-87 2.0L Lynx, Topaz Diesel
1985 2.2L Ranger
1989-92 2.2L Probe
1986-88 2.3L Ranger
1986-87 2.3L Diesel Ranger
1991-98 4.6L Crown Victoria
GM
1986-95 1.0L Geo Metro
1989-91 1.0L Firefly (CANADA)
1985-88 1.5L Sunburst (CANADA)
1985-89 1.5L Spectrum
1990-93 1.6L Prizm, Storm
1981-84 1.8L Diesel (CANADA)
1982-86 1.8L Buick Skyhawk
1990-98 1.9L Saturn
1987-88 2.0L Buick Skyhawk
1988-95 2.3L Quad Four
1985-87 3.0L Buick
1979-95 3.8L Buick
HONDA
1986-87 1.0L Prelude
1973-78 1.2L All
1973-78 1.3L All
1980-84 1.3L All
1973-78 1.5L All
1985-89 1.5L Civic
1988-95 1.5L Civic, CRX
1993-95 1.5L Civic Del Sol
1979-84 1.5L All
1985-87 1.5L CRX
1993-95 1.6L Civic Del Sol
1973-78 1.6L All
1980-82 1.6L All
1988-95 1.6L Civic, CRX
1984-87 1.8L Prelude, Accord
1979-83 1.8L All
1986-91 2.0L Prelude
1990-91 2.1L Prelude
1990-95 2.2L Prelude, Accord
1992-95 2.2L Prelude
1995 2.7L Accord
HYUNDAI
1984-95 1.5L Excel Scoupe
1995-98 1.5L Accent
1992-95 1.6L Elantra
1993-95 1.8L Elantra
1992-95 2.0L Sonata
1989-91 2.4L Sonata
1990-95 3.0L Sonata
INFINITI
1990-92 3.0L M30
ISUZU
1987-89 1.5L I-Mark
1990-93 1.6L Stylus Impulse
1987-89 2.0L Impulse
1981-87 2.2L Diesel Truck
1986-95 2.3L Truck Trooper
1988-95 2.6L Truck Rodeo Amigo
1991-96 3.2L Trooper Rodeo Amigo
KIA
1995 2.0L Sportage
MAZDA
1984-85 2.0L 626
1988-92 2.2L 626 MX6
1989-93 2.2L Pickup
1988-95 3.0L 929 MPV
MITSUBISHI
1985-95 1.5L Mirage Precise
1990-92 1.6L Mirage
1989-95 2.0L Galant Eclipse
1983-86 2.3L Diesel Pickup
1994-95 2.4L Galant
NISSAN
1982 1.5L Centra
1983-88 1.6L Sentra Pulsar
1987-89 1.8L Pulsar
1982-89 2.0L Stanza 300ZX
1984-95 3.0L Maxima 300ZX Pathfinder
PORSCHE
1976-83 2.0L 924
1976-89 2.5L 944 Series
1989 2.7L 944 Series
1989-91 3.0L 944 Series
1976-83 4.5L 928
1984 4.7L 928
1985-91 5.0L 928
1992-95 5.4L 928
SUZUKI
1985-94 1.3L Samurai Sidekick
1989-94 1.3L Swift
TOYOTA
1986-95 1.5L Tercel
1981-83 2.2L Pickup
1984-87 2.4L Pickup
1982-88 2.8L Celica Cressida
1987-94 3.0L 4-Runner
VOLKSWAGEN
1976-91 All Except 1.9 2.1L Engine
1990-92 1.6L Golf (CANADA) Jetta
1990-95 2.0L GTI Jetta GLI Passat
VOLVO
1991 2.3L Coupe 940
1986-94 2.3L 240 740 940