Can the venerable RD350 engine go in? And would there be much point? The extra cubes give better torque, but with the TYGA 300 kit, I guess there's little in it.

What about the Rotax 450cc two stroke engines they use in Skidoo's?
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MC21SP Plaything
BMW F800GS Bumblebee
Triumph 9551 Daytona Big boys toy
FJ1100 Sporting relic
GTS1000 oddball