You'll have to check but I think the aftermarket heater cores are aluminum. And it's a bear of a job. Basically the entire dash has to come out to access the HVAC box. And you might as well replace the AC evaporator while you're in there, but that will require a licensed AC tech to recover the freon and you may have to convert over to R-134a since R-12 is very hard to find.
When the heater core went on my '92 Acclaim, my fix was to temporarily T the heater hoses together (not fun as it was middle of winter) and I eventually replaced the '92 with a '90 Acclaim.
When the heater core went on my '92 Acclaim, my fix was to temporarily T the heater hoses together (not fun as it was middle of winter) and I eventually replaced the '92 with a '90 Acclaim.