It's getting harder and harder to get smaller 4-cyl RWD engines that suit conversions.
I liked my factory injected 4AGE Morris, if it fits in a Morrie, then should be easy in a Viva, but as noted above the RWD aspect of the 4AGE is getting more complicated and expensive.
How about a Zetec, they seem to more convertible to RWD, and are more current I think.