If I may speak for the OP for a moment, no matter the engine platform he will need to remain turbo. Remember aircraft reach high altitudes where the air pressure drops. Every 10k feet is like 2/3 the power at sea level. A low boost turbo will keep the engine from "leaning out"
I think the most important question is "What engine management will he be using"
If he can go aftermarket my vote goes for EJ20 since they are a dime a dozen.
My second vote goes for an intercooled low boost EJ25. A 99+ ej25 will probably already make 170hp since he will mount a custom exhaust and even 18psi manifold pressure (+4 over sea level) would probably put it near 200 with an intercooler. The RS25 guys have plenty of experience turboing the EJ25 before the 04 USDM STI came out.