Jim,
First off, let me tell you that I've never even seen an RM100, and I have no idea if the 125 top end will fit - so I waited to reply in case someone else had specific experience with this mod. One thing I'm fairly certain of, though, is that the RM125 piston will be heavier - so the crank will be out of balance, probably resulting in substantial vibration.
The route to more power is pretty much the same for all internal-combustion engines:
1) Get more air/fuel in 2) Compress it more 3) Ignite it better 4) Extract the burnt gases more efficiently
A bigger carb - perhaps the one from the RM125 of that era - may bolt right on, and should give you more top-end power. If you take the time to jet it correctly, the loss of low-end will be minimal.
More compression can also help with low and mid-range power, and adds 'snap' coming out of corners. The best way to do this is to have a machine shop mill material off the top of the cylinder, or use a thinner head gasket if you can find or make one. .010" off the cylinder top or gasket thickness is a good starting point.
If you can find an aftermarket exhaust or reeds, that would be great, but it probably will involve a hunt on eBay, Google, etc. The bike already has an electronic ignition, so that's good.
Ray
_________________ '74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric
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