I am in the process of rebuilding an MC21 engine to put into an MC18 R2J frame. Research on this site indicates there will be no problem fitting the MC21 engine into the MC18 frame. I am aware that the MC18 harness has wiring for a neutral switch whereas the MC21 engine will have a gear position sensor (GPS). I will be putting a blank plug in the GPS hole. The neutral light will not work. The wire splice for the MC18 is the simplest as there is no GPS. Not sure of other wiring issues like throttle position sensor (TPS) as i haven;t got there yet.

I don't see any problem with carbs and pipes either.

However, the crank on your MC18 can be rebuilt. New bearings and seals are available. Connecting rods and gudgeon pins are not though. Use the search feature on this site and you will find much information on rebuilding cranks.

Cylinders can be replated or new ones for an MC28 will fit and are available too.

Paying a mechanic's rate to rebuild an engine could be quite costly. You may want to take up the challenge and do it yourself. It's not too hard and you don't need much in the way of tools. But you will need time, patience and the desire to learn.

Or contact Tyga. They can help with a rebuilt engine. And have much experience in doing so.
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MC21R9N track
MC18R2J track
CBR954
CR250