Here is a thread I started about Homelite 25cc modifications. A lot of it will probably not apply to your Mitsubishi, but the part about lowering the cylinder will.

Click here: http://www.rcuniverse.com/forum/engi...c-hot-rod.html

Read through all of it so that you understand that lowering the cylinder to increase compression will decrease your exhaust and transfer port timings. They will need to be ported, or at least the exhaust will, to gain back the lost timing. I often leave out the cylinder base gasket to increase compression, and just use a good sealer instead of the gasket. You must be sure to check the piston to head clearance though.

Whomever told you a higher compression ratio would help at high altitudes was correct. You can also try advancing the timing a couple of degrees.

AV8TOR