you probably don't need the pump,its nice to have but not totally necessary,it keeps the fuel coming at a constant rate, so in high G turns or when doing 3D type flying you get good fuel pressure.
also you can mount the tank on the CofG and and the pump will draw the fuel from there instead of needing the tank behind the firewall, I have several pumped engines there fine,but the pump doesn't add any real power just fuel flow, my Marie has a standard non pumped OS surpass,I also have a H-9 Warhawk with the same OS 91 it flys great.