Looking through the old articles there are a few articles about the 74-76 MX125, I am wondering if anyone has converted a DT to an MX. Do you need to do cylinder and head together or can you jut do the cylinder? Would it even make sense if you kept the original exhaust and carb? Doing a bunch of stuff on my 74 DT125 and the top end is worn out (75psi compression) so just trying to get an idea of what is possible. Kind of wanting to mess around with the 125 for practice before ding the 175 swap at some point.

Pretty high strung little engine.  Definitely need head.  You'd also really need at least pipe and probably carb.  You could use YZ too. 

It'd be expensive to find those MX parts I expect...  DT175 top end is a lot cheaper and much better power curve.  There's a lot of 74-76 MX and YZ 125's running DT175 top ends or engines.  With mild professional porting the DT175 engine will absolutely run away from the MX125 of that era.  And be a lot more fun to ride.  Just my opinion.