88~97 (MC18~MC28) motors share similar crankcases, with the same frame pick-up points, meaning that any motor will fit any frame. (The MC16 motor has the same pick-up points, but has solid mounts and isn't a direct fit in any other chassis.)

The best porting of all models is the 1989 R5/6K.

As geezagillard says, it's really not worth you damaging braincells worrying about porting/compression/ignition timing etc., as it's going to make no noticeable difference to an otherwise standard bike. What you do want to worry about with any kind of mix'n'match however, is that the RC Valves operate correctly. Make sure that once it's all up and running that you [a] have the required slack in the cables, and [b] have full sweep of the valves. Not all RC Valves are created equal! This may mean you need different cables or servo, or both, depending on your final choice of components!

At the end of the day, you're just going to have to make a decision, jump in, and figure it all out! Most people just rebuild or source the "correct" replacement engine, so you are asking questions most people don't know the full answer to!
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Andy.
NSR-WORLD.COM

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