Well there are ways of modifying a diecast kit. First off you will need to disassemble the kit. You can use paint stripper to clean off the metal body of all paint. Make sure to prim it before repainting. You can modify the body with JBWeld and a dremel tool. You can cut the body, add to it, open doors and such just like a plastic kit. I recommend you start small before taking on a 1/24 scale metal kit. try modifying 1/64 scale Hot Wheels and work your way up.
Scott