I just purchased a honda cb250 1975 no spark

Make sure battery has a full charge. Cranking is starter motor action, meaning the starter motor is cranking the engine. If the starter motor won't crank the engine, then we need to test starter motor wiring circuits.
If it cranks good, but, won't start, have a helper crank it while you check for spark at the spark plugs, where applicable. If spark everywhere, use a gage and check proper fuel pressure and also, check for fuel injector pulse, engine cranking.
Can you check for applicable trouble codes?
I can only make suggestions, you have to do the testing. I give the same info to guys and gals, I don't know how to do it any other way.

I'm sorry but. Not that I know of. .i feel for ya .that sucks

You checked for spark during cranking?
You checked fuel pressure at the motor?

Explain what you checked & how & provide
test results

The motor turning over & cranking---- is one in the same

You have a :
NO CRANK or a NO START !!!

You checked fuel pressure at the motor
You checked for spark or injector pulse
The battery is good-you tested it

Now tell use about the loss of ignition output

Where is all that coming from--in detail

Have to crank sensor scoped

Is the ignition switch worn out ?

does the motor crank , are you grounding the spark plugs close enough ? check your coil, . try new plugs (cheap easy option) ... check your earth from the coil , check all your earths , clean , dry and tight ...

If the starter is cranking but not starting, your coils have gone bad. They are what plugs into the top of the spark plug. Isuzu Motors/Honda combined the coils to make it simple but in the process made things less reliable. If no sounds are coming at all, not even the chime when you open your door with the key in, replace the battery!

you better check the starter motor relay

In short no there is absolutely no way to bypass a sensor that picks engine rotations.

no you cant need a crank sensor

Next things to check are fuel and spark, remove a spark plug and with the wire attached crank the motor, if you see spark in the plug then it's not a spark issue and probably a fuel problem. If there is no spark could be bad plugs or wires, or distrubitor cap. To check fuel remove injector cover and crank motor, if there is no fuel coming out then check fuel filter, fuel pump.

Good Luck