I would say, yes, you can build a carbon fiber pan rat if you want.
I would define the engine as the crankcase, crankshaft, piston, cylinder, head, rod, and wrist pin. If I was running the event, you could grind & polish these, but not replace them.
You could go wild on the backplate, gaskets, bolts, and NVA.
The speed is ultimately capped by the fuel, suction tank, and the strength of the Fox parts.
I'm hoping to run a few tests tomorrow to get an idea of the times possible with off-the-shelf sport planes.
I'm not convinced that airplane size is all that important in the sub-90 speed range.