buy an older leaf front chev truck, just shorten the frame and driveshafts, and youre done. remember though, if you wheel, all the extra weight is going to make you sink faster. its also harder to climb up steep hills, and youre really giving up the trackers advantage.
I only heard of aftermarket aluminum 60deg 2.8 blocks, dont think factory ever produced other than cast iron
a quad4 2.3L is 180hp, steel block though, can take a turbo safely to about 300, 400 if you know how to use it.
Land rover has a 3.5 aluminum V8, same as the early buick/olds 215 motors, about 220hp if you can find one.
RX7 rotary engines are heavy for their size, 400# without turbo. no bottom end torque, so it might suck on the trail