Depending on when in '97 it was made, that engine could have either the standard GT40 heads, or the GT40P's. Either way, you'd probably want to go diving into the engine replacing valvesprings at a minimum, as those aren't performance springs, and really wouldn't support very many aftermarket cams. If you can justify the cost of $1200 on a used engine, I say go for it, if you want a "new" engine, but keep in mind that you may end up paying a fair amount more going with a new cam, valvesprings, lifters, rockers, etc. You could swap the engine as it sits now, just using your distributor instead of the distributorless ignition on the Explorer engine, and give yourself time to save for future mods. Just my opinion, but I'd put that $1200 toward rebuilding the stock engine, or buying heads, and saving for more later.