newbie, 1975 scamp

Just picked up a 1975 scamp for a project car for my son and I. It came with a slant 6 with the upgraded intake manifold holding a 4 barrel carb. Has the split gurgle pipes and has been lowered in the front with new shocks in the rear. I'm not sure if I should rebuild the 6 or drop in a 318. would like to build a simple muscle car without doing a complete body off restoration.
Parts seem to be a challenge here in B.C. and don't really want to buy a doner car.
Any suggestions ?:violent1: