Jeremy,
On the early Viper you need to engineer the space for the fuel, and turbine accessories-ECU/solenoids/fuel pump/etc. A belly/ventral tank would be a hassle with the bolt-on wing from underneath although it may be possible for a one-piece model. I had to fabricate extra hatches on my Cyclone conversion. I have an early Aggressor I/II which I thought of converting to a Wren 44K but it was just too much hassle. Also, have a BVM EVF/10S Lipo system but I'll probably fly the model as a glow DF version as a nostalgia jet.
Rgds,
Art ARRO
.