Aftermarket carb which might need a custom manifold.
Aftermarket exhaust which might need to be custom.
Advanced ignition timing (throw a few degrees at it). Guessing it is set by the flywheel position. Probably uses same key as GX160, GX200, or you could just lap it in.
I'm guessing the teeth on the timing belt are similar to the teeth on cam/crank gears. 1 Tooth = 15 degrees of crank rotation?
You could try to retard the cam timing, but I think that would be too many degrees. This would push the powerband to higher RPMs.
Similarly, you could try to advance the cam timing. This would keep RPMs low which would be better for stock parts. This would also build high cranking pressure without milling the head or using a thinner gasket (since that is not possible). But again, if it is 15 degrees per tooth, then that is a pretty big jump and I don't know anything about the cam timing on this engine. Might be interesting to try? Throw some high octane gas in it and advance the cam a tooth?
You could shim the valve springs to get a higher pressure and maybe get a few more RPMs out of the stock springs if you hit a valve float.
Tighten up the valve lash to get the max lift and duration out of the stock cam.
You might be able to modify the rockers somehow to get a higher ratio.
Do the cam lobes hit the rockers under the valve tips? And the "L" shaped rockers pull the valve down? I guess I thought the cam was going to be OVER the valve tips.
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Those are the ideas I had after looking at a few pictures.
I'm guessing valve job and porting might be difficult?
Not sure I'd want to do too much to an engine I'd be afraid of. Stock parts + Modified = Scary