probably just gona go though the motor I have and find a head.
Looks like you managed to have some fun despite the vestiges of the shit missile giving you some grief.
What are you going to do with the engine, leave it the way it is for now, replace the head or swap out the engine? I like JB Weld as much as the next guy but don't think I'd trust the stuff quite that far.
I know put a turbine in it if you're looking for something light. Think of the cool ass sound it would make and you don't see that shit everyday either.
Failing that scrounge up a Mazda Wankel, that would be small and light and it should drop right in there with room to spare. A 13B shouldn't be too hard to find. 410lbs including twin turbos and all accessories, PS, AC , airpump etc. IIRC. You could drop the AC and save a little weight for Keystone instead.
Dare to be different but in a different way than you're different now.