Last year I did a complete paint job, so while my memory is fresh, a couple of advices:
Engine enamel will be cured with heat from the engine (do not cook it too high, though). If you used the proper stuff, you're good to go. I used Motip Engine Paint, it seems to hold.
Tank paint: Did you use simple 1k acrylic paint from a spray can? There's a chance it never really will be fuel resistant. You can try to overcoat it with clear spray lacquer, but it will not adhere well, if it's not done while the original paint is fresh (ie. clear coat it in 1-2 hours after painting). If the original paint is non-metallic, you can roughen it a little with sanding (ie. 800-1000 grit) and then overcoat it with clear.
Proper solution is a 2k car paint, either from a spray can, or applied with spray gun. That will be cured properly and will be totally fuel and oil resistant in couple of days. Obviously I you want metallic, that will be a base + clear coat job.
One more thing: I've seen 1k acrylic paint sprays labeled "2k". The proper 2k (2 component) paint is 1 part paint + 1 part hardener (non 1:1 ratio in reality), which must be mixed before use. In spray can format there's a little red cap, which contains the hardener, you mix it in by pushing the red cap.
The fake 2k paint (or simple 1k acrylic paint) is 1 part paint + solvent, which will evaporate on apply, but these paints won't be "cured", instead "dried" only, hence the low resistance against chemicals.
