I'm pretty sure unstablehybrids.com liked the rb20 engine since they did a few swaps with it for their project cars. This may only be because it is a cheap engine in comparison to an rb25 or rb26, but maybe it was for ease of modification or something else.
MR2 is a great car. Very reliable, great handling, huge performance potential, etc. They do pretty well in amatuer racing. You can get a 91 for a very reasonable price and you don't have to do an engine swap...though it can be a good idea for more potential.