Replacing the 18RG is obviously the easiest thing to do,O Man RA23 has an 18RG for sale. 18RG for sale
Any other engine will more than likely be EFI and more powerful which means at the least, upgraded brakes and wiring, engineering, potentially diff and tailshaft etc, the list goes on.
The cheapest engine upgrade is a 1G of some description (either N/A, turbo or supercharged). This particular engine swap has been done a million times so there are plenty of articles out there and heaps of people to help.
Hope that helps.
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