Hi Rob,
Renli and Goka parts are sometimes hard to come across!
A car engine transplant would be a big job, the back end of the named buggies would not be strong enough to take a car engine, unless you go for the Goka 1100 (which runs a Suzuki Alto engine).
You'd not need to change a Goka 1100, as they are ment to be quite nippy ;D
I've been in a Traveller 1100 (superbyke) and that performed very well!
Keep us informed of your developments
Steve.