The tonerider rebel 90s are pretty good humbucker-sized p90s for the money. Only come in nickel finish, though.

Nothing ever "just fits right in." Especially with a Gibson or Epiphone. But it looks like it might make a nice guitar - one of these days... I have two '60s vintage Gibson SGs - they've never been easy to live with. But I will say - both have P90s, and I wouldn't have it any other way. Both left the factory with P90s, but one (a Junior) had been modified by the time I got it, and had a humbucker in it. I used it like that for a long time, but finally had to put a P90 back in. My retrofit is not cosmetically pretty to the trained eye (or any eye that takes a careful look), but it sounds so much better now - to my ears, anyway.

Not really Epiphone's fault that P90's don't fit in a guitar designed for humbuckers.

Guitars are so stupid 😂 sometimes it's a fight to get it right 👍