I've never heard of a plastic MJ tank. the late 90s Dakota would have a plastic tank that fits, but the pump stuff would be harder to adapt. the link in my signature would have a couple writeups on guys that used the Dak tank, but they were in conjunction with 97+ swaps which better matched the dakota pump style.
when you get your new tank, do a scuff, chemical clean, primer and a couple coats of paint and it should last a looooong time.