Vintage Dirt Bike Q & A

Will a 1974 CR125M cylinder fit on a 1976 MT125 lower end? If so, I was wondering whether you could machine the cylinder to accept the intake for the oil injection. If it would fit, I'm expecting that cylinder with the larger carb would increase the performance with the advantage of retaining the five-speed MT transmission. I have the parts from years of collecting and was curious. I use the bike primarily for riding on the street and dirt roads.

Yes 74 CR cylinder will fit on 76 MT lower end.
The MT cylinder has a thicker built-up area at the tapped hole where the oil injection bolts in. I think the CR could be carefully drilled and tapped even with a thinner wall, but be careful not to strip it.
The CR carb will help performance. A CR pipe would help but is not practical on the street. A low restriction air filter and air intake routing will help performance as well because the MT airbox and filter are pretty restrictive. For the street you need the battery, oil injection tank and electrics that are taking up space where the CR's airbox would be so you can't do much.
The CR cylinder will make the bike's powerband peaky like a CR. The CR has very low power outside of that very narrow powerband. This may make the MT transmission a problem. It spreads 5 gears over a ratio range of 2.678. The CR transmission spreads 6 gears over a smaller ratio range of 2.424. When you are riding the bike and accelerating on a CR each gear change only drops the rpm a bit. On an MT each gear change drops the rpm a lot. With the MT transmission it could be difficult to ride because when you shift up to the next gear, the engine rpm may drop below the powerband and struggle to accelerate the bike.
Give it a try. Good luck.