I picked up an old 1990ish Norco TNT mountain bike that I want to rebuild. I want to keep the original paint and decals but I was thinking of doing a tremclad clear coat. Has anyone done this? Good idea or bad?

If I remember my time up North correctly tremclad is just enamel designed to bond to rust. Probably fine if you want to control rust formation under the clear, to the extent that is possible. In my experience if you have rust some where under a coating it will eventually bubble out. That said I have used the new rust resolver products that chemically change the rust and they to date have held up on the trailers I applied it to. If you don't have a rust problem I would research USC SprayMax 2k Glamour High Gloss Aerosol Clear. It's $20+ for a can the last time I bought some and a two part that looks good and is much tougher than any enamel or lacquer.