Have you considered buying a glass (or plexi) cocktail table or end table that you can put art inside like a big shadow box? The tank might fit nicely in a end table. You'd need more memorabilia to fill a cocktail/coffee table but it would look good.
Another idea is a lamp of similar design. DCFiremann can send you pics of his lamps that have birds mounted inside with backlighting for display or a nightlight. They actually look pretty cool. Making them with Harley memorabilia would look fabulous. I've seen the plaques that you've mentioned and the miniature versions look good. I guess if the tank didn't cost that much and you're pretty sure you won't ever have a need for it, the plaque is a good idea. But geez, they are limited in number. If it was me, I'd probably mount the wooden frame around the whole tank and then knock a dadgum hole in the wall so it could be mounted flush as if it was only half a tank before I'd actually cut the tank in half. Heck, patching the drywall might be cheaper than replacing the tank! How big is the ding in it? Tried popping it out like a hail-dent on a car? I'd tell you how I've done that on my cars but if it doesn't go well, I wouldn't want to take the blame and your bike is worth a lot more than my Honda or Suburban! It might not work anyway because it's not as flimsy as a roof or hood of a car.
Another option is picking up a plain, non-CVO tank that is damaged on one side and just have your guy split it and make you a replica of the red tank. I'd have him do it first to see if he can truly match the goldleaf exactly. Then continue the theme by displaying the full tank in the bottom of a table, bar, display case, lamp stand, or whatever in your Harley room.