Don't use the fiero. The last year of production was the only year to actually get update sports suspension redesign. Every year prior used cheap, crappy GM parts bin stuff from the chevette. Shitty brakes, awful geometry, bad chassis. Terrible engine. Not to mention an inherent problem with the fuel line leaking and causing a fire.
The mr2 is prince of the auto cross(Miata is king). No reason it wouldn't do well for the dirt. It's well documented, bulletproof, looks good, plentiful. And engine swaps don't cost thousands.
Side note: can't believe I forgot the link... I'm losing it.