I have run the stock Fox 35 and a few Fox 35s with modifications. Years ago, Fox had a deal going. If you sent them a worn out, beyond repair engine, they would send you a new one of same type for half price. So, I got some new Fox 35s and used the savings to get the ABC kits, hemi heads, and stuffer backplates. As far as I can remember, all of those new parts were factory supplied items.
The only difference I noticed is that with the ABC kit, which contained a lighter piston, vibration was reduced. I noticed no difference with the hemi head and stuffer backplate. Power appeared to be the same. If there was a difference with the new head and stuffer backplate, I could not see it. Maybe too small of a change to notice.
I have no experience with any other performance mods, except for using an ST needle assembly if the stock unit proved to be a problem.
With the ABC kit, you might be able to reduce the castor content of the fuel. But, that crank main bearing is still needing the same amount of oil.
Fuel consumption was the same. That is to say, terrible, since so much of it went out the exhaust to help cool the engine.
I ran these modified Foxes when I got back into the hobby in the late 90s. I did not know any better so I went with what I had. After that, when I found better engines, I pretty much gave up on the Fox. It is outclassed by modern hardware. And for the money spent on modifying one, you can get a much better engine for less money.
Now, I wish I had bought another Brodak 40. It is far better than the Fox in every respect. Another bargain is the Magnum 36 but try finding one now.
And, with electric taking over, the Fox is pretty much a nostalgic item. I still have them, and one 50th anniversary version that has never been run. Maybe one day I build something for it, just for memories.