Great question! Will not fit a pre-1978 125/175. Questionable quality from what I've read. Since it won't fit, it doesn't matter.

You need a 72-73 CT or 74-76 DT175 (matched) cylinder and head. If outside the US, others might work but those are US models.

Basically 175 reed valve twin shock models... not monoshock. Listed cylinder is a 78-81 monoshock cylinder.

Highly respected tuners and engine builders Dave Meis and DVM have recommended the 74-75 DT175 cylinder/head is the one they prefer.

P.S. if you source a 1976 cylinder/head you'll have to switch oil injection port to carb. Very easy job. 1976 was the first year of the oil injection to the carb instead of base of cylinder on the 175's

Thanks for the advice! I'm actually going to be doing an oil injection delete and running premix so maybe a 76-77 would be best?

No 1977 DT175 model in the USA.

Won't matter much, a small bolt will block injection port at base of cylinder if you bypass oil injection.

One last thing, will it require a different spark plug? And isn't the ignition on the 125 12v and 6v on the 175?

Parts lists show that both bikes use the same B8ES plug.

Changing barrel and head shouldn't affect voltage system, or vice versa.

Yes, you are correct on the US models the twin shock 125 enduro was 12v generator/starter and the 175 was 6 volt magneto.

The only "negative" is that the electric start might not have enough "ummph" to crank the engine (especially when cold) after converting to a 175.. which is likely why Yamaha didn't do it. They kick start real easy though and it doesn't matter which ignition you use as long as you have spark.