The standard fuels tanks are 40 oz. + 16 oz. You could get a bit more in with conformal tanks.

I don't think a P-80 would be that easy to install. It's OD is almost one inch larger than the intended engines.

A P-70 will fit, and gives almost the same thrust as the 80. Engines of the intended size, such as the Simjet 1200 SP and Wren 54 SS (when it arrives), are not far behind either. All three engines will give greater than 1:1 thrust to weight with a reasonable build.