1981 Yamaha DT125 no spark

SOURCE: fuel for yamaha dt 125

This bike uses unleaded gasoline. At only 6.7 to 1 compression you can use the lowest octane fuel you can find. 87 in the US is what that would be around here, but across the pond the octane numbers are different. But don't forget the two stroke oil. That may be more expensive than your fuel cost.

SOURCE: a lot of gas coming out of the carb. overflow tube, eng on or off

the float pin is stuck or has some dirt in it. try tapping the side of the float bowl(s) with the handle of a screwdriver... not hard enough to break it. if that doesn't work you will likely have to take the carb apart and clean it.

SOURCE: what spark plug for yz 125?

I have a dennis kirk book, the correct spark plug is a --BR9ES--(NGK)

i always used SAE 10w-30 oil on all my dirtbikes,
no transmission fluid

for the chain you should move the back tire back until the chain is at a snug tension

SOURCE: I have a 95 yz 125 head and 98 yz 125 bottom end.

You have a 50/50 chance that they will work okay. All the mounting holes need to line up and the cooling holes as well. Get a head gasket for the '98 and see if all lines up. Will the exhaust pipe fit the '95?

SOURCE: headlight 1972 yamaha 250 yz

You can add a headlight if the type of ignition system on your particular model is the type that will let you. Several different systems were used back then. The proper system will need a lighting coil installed onto the plate underneath the flywheel. The flywheel must be pulled off. You will see a coil there already for the spark. An unused set of threaded screw holes is present, (the same plate was used on the bikes that had a headlight). Just add the coil and now you have power for a headlight.