The heater core stays the same, but you might as well check the one you've got in there to make sure it's in good shape. If it looks corroded replace it while the heater assembly is out. If you do take the back cover off the heater assembly you'll need to run a bead of RTV sealant when you put the cover back on.
As for the replacement motors being as good, I don't really see how. When I installed my Blazer motor I needed to cut the hole in the firewall bigger as the Blazer motor was much larger than the factory sized hole. I cant see how a replacement motor could fit in that hole and still be as powerful as the much larger Chevy motor. Just my thoughts.
'87 YJ Laredo
- 4.0L, AX-15, NP231, 4.10s: 3.5" BDS, 31" Muds
- Donated drive train to the YJ, Parted out the rest
- Buick 225 V6: Painless Wiring Harness, Warn Overdrive & Hubs, 2" Lift, 31" Muds