There are people who have put "window" doors from Gen II Vipers onto Gen I cars. And I think you can a lightweight engine into your Viper, but... I had a 93 RT, and wanted the new wheels, hardtop, and AC. When I looked at the cost, I could have upgraded to a newer car. So I did.
If you need to replace the engine, or find a cheap engine or set of doors, it might be fun to do. In general, you don't get your money back on modified cars, Vipers included. You might want to price it out and see what a Gen II roadster would cost you compared to the upgrades.