Its alot of work, a 300 doller 1990 125 motor will probably be as bad as what you have. Its sad but most old mx bikes (95 and older) end in the junk yard because they cost way to much to fix. some cases the price to fix is well over the cost to replace.
Long story short if you want a nice mx bike you really need to go big or go home. $150 mx bikes are junk. In like 03 I bought a 96 yz125 for $1500 what a sweet bike that was!!! But even as nice shape it was in Between My dad and i there was much work and money put into it Replacing piston and rings, Renickling the cylinder, new crank.....THATS ALMOST $1000 bucks there!!
At 14 I rebuilt the transmission in my old yz. Too many times shifting while wheelieing without using a clutch. The gears and fork in the trans was almost $500 to replace and i was doing my own work.
