In low rpm applications like aircraft, conservative timing is advisable, power gains will come from just making the ports wider where timing changes will move power band up in the rpm range.
Either a timing change or port width change will make an impact on time areas.

If you are using a lawn tool engine the stock timing usually is about right for aircraft.If you want a little more rpm increase exhaust and intake timing a few degrees and try it.
But start with wider ports.Leave the transfer port timing where it is but widen ports.

The exhaust port will give you the single bigest gain.

Pipes, boat guys use silicone tubing to connect pipe to header. Cone angles are important, again conservative is best.Making pipes is a pain in the neck. You may find a usable pipe from the goped hotrod parts that you can adapt.
One of the boat guys made a pipe from an aluminum baseball bat jb welded together for a boat. It may not be big enough in diameter for your size engine.