I'd be rebuilding it, the motor is gone in it so hoping to get it fairly cheap. Throw some pistons in it from a 93+ 2.6L and a new pre 93 head and it'd drop compression to about 7.5-8:1. slap some arp rod bolts and head studs and it should handle a decent bit, nothing crazy. Alot of guys run the Impulse turbo setups on em' without rebuilding, but ya gotta be careful with timing etc.

I think I know the stuff your talking about, ya it's "properties" aren't the same as the metal in your door so that's why you have the color difference, it absorbs the paint differently. I've got a few friends with welders down near you if you decide to try welding a plate in.