The manifold & engine I linked offer two different solutions, you wouldn't use both.
The manifold I linked would allow you to put a side-exhaust engine, like the SH.18 in that vehicle, though you may need to fit a roto-start instead of a pullstart.
The engine I linked is identical to the one...
You could get an exhaust manifold that wraps another 90 degrees from the side, like this one, so the pipe attaches in the same place. There's also a few rear-exhaust .18's left on ebay, could drop one of those in directly.
Elbows have been attempted but tend to be a failure point, a straight...