1 and 2)chainsaws are generally ported for more power than a weedeater engine, but most chainsaws are large (30CC+) and most of the small saws have roller main bearings that don't lend themselves to thrust loads as well as ball bearings. For a trainer type plane as long as the power is reasonable it probably won't matter, but if you want to tinker with porting the cylinder you can wake up a weedeater engine.

3) that ignition should work just fine