Brief:
The problem with overpowering a "stubby" design like the C-powered Estes Fat Boy with heavier engines is the CG dilemma -- the more power, the more nose weight is needed to keep the CG in limits, negating the Oomph you thought you were adding. I have modified Fat Boy for F-power with a nose cone extension to counter-ballast this with minimal added weight.

Modifications:
The only other modifications I made to create my Schnoz Boy was some modest reinforcing to accommodate the 8-fold increase in thrust (C to F).

The stock fins were trimmed to fit a 24mm motor tube (or a 29 mm works, too) and doubled on one side with 1/64" sheet plywood (a snap, this stuff cuts with a scissors) epoxied on. A 1/8" Lite-Ply-tm doubler was constructed for each of the cardstock motor centering rings supplied with the kit. And that's it. To complete the make-over, I added a "professional" recovery system consisting of an 18" LOC nylon chute and 10 feet of nylon Marine Corps shoe lace (see landing photo), anchored through the body tube with a knot covered with a plastic fairing. Copying a final touch I saw in a magazine, I couldn't resist rearranging the stock Fat Boy decal to spell "YO BATF".

Construction:
A 5" 24mm Estes nose cone was ballasted with 35g of lead fishing sinkers epoxied into the tip. The "supplemental" nose cone is then affixed to the stock Fat Boy cone, using a self-tapping sheet metal screw set into the stock cone and protruding 1/2" to allow a gob of epoxy to anchor the pointy ballast box in place. This puts the CG with a 62g F21 at 6.5" from the tail, well ahead of the Cp (3.9" from the tail). I call the finished product "Schnoz Boy", for the obvious reason. See photo.

This was an easy conversion. All the stock pieces were used, and its a great kit. The through the wall fin design makes this easy to convert to higher power. You do need to add a proper recovery system for E-F-G power, however, and reinforcing the balsa fins for the much higher speeds is essential. I know over the years many folks have had fun overpowering Fat Boys with G80's and H180's and even more thrust -- but seriously, how many of these plywood-finned, fiberglass-reinforced cardboard mortar shells do you ever get back in one piece? Schnoz Boy is a spectacular flier on F-power, and you get it back to fly again and again. It makes for an interesting drag race when you sneak it alongside some kid's C6-7 Fat Boy on the club launch rack.

Flight:
How does Schnoz Boy fly? On E30-7's, it roars up to about 800-900' in the blink of an eye. On an F21-8, it rides a foot long sheet of flame to at least 1700', with ejection about at apogee (not that you can really see this). I have several F24 reloads I plan to try in it. A good first flight was had on a D12-5, but Schnoz Boy is too heavy to fly well on an Estes E9-6 (meaning that I tried, and I don't want to talk about it). After 11 flights to date, not a bit of damage (except to the stock 18" Estes plastic chute, which shredded on the first E30 flight, dropping Schnoz Boy onto a sand dune). The only shortcoming with Schnoz Boy lies in the future of Aerotech as sole supplier of single-use 24mm/29mm E-F-G motors, without which it wouldn't be much fun.

Summary:
PRO: This is the way to fly an Estes Fat Boy for "Big Kids"!

CON: Well, of course, you lose the ability to fly on C6-7's, and unless the flow of AP motors in 24mm size restarts, this conversion is too much trouble for just a D12-5 upgrade.

Other:
This approach to changing the rules for ballasting -- moving the nose weight ahead of the stock nose cone instead of inside it -- has a lot of possibilities for balancing rockets of all sizes with minimal added weight.