As a big advocate of Ultra (to the point where my HVZ power games involve an Ultra Amp), I think you'll struggle to find many folk willing to mod Ultra to continue to fir Ultra darts.

Most ultra mods swap out to short darts, particularly the flywheelers.

My best recommendation is the 15 dart extended mags you can get from China (Ali express IIRC), they need a spring adjusting and the follower lubricating, but they work fine afterwards!

Edit: spelling