janet what is wrong with the engine? what have you been told? it could be algae in the tank and a very simple fix... just buy algae killer at at truck or tractor supply and dump it in the tank and let it kill kill kill.
the swap from 123 body to a 115 body is possible but has many incompatablilites, better to get a same body and same year to switch over.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.