If you were going to rebuild the engine, you should just get a N1 crate engine 13k i think. Drop it in and you're off. Dad's crate engine has done 3 Targas, and some open circuit racing, and it's still going strong. The power has ranged from 330rwkw to 400rwkw over the years.
I think you'll be fighting a uphill battle to run an external pump. As the GT-R has been proven realiable fast car now for years with a sump and good pump.
The standard gauge in my R32 GT-R does the same as yours, it's as good as usless. gauge shows no pressure, turn it off and on, and you magicly have 4 bar again.