I believe the backhoe BT-900 was the factory OEM backhoe fitted onto the L-35 tractor loader. The mounting in that application was to the tractor frame.
I'm not aware of BT-900 fitting any other machine "as configured" for the L-35. I'm pretty sure the L-35 was a 'one off' model.
With that said, if you can figure out or scratch up a mounting process or frame members fitting other tractor(s), it will probably fit any machine that can carry it. The problem will be 'properly' frame-mounting it to some other machine.
There have been threads on this Forum about sticking backhoe units onto a 3-point lift frame (which I wouldn't recommend for the -900 on less than about a 40-hp tractor; it's heavy). I've even seen a couple trucks where somebody mounted a backhoe on the rear bumper. Unit doesn't care so long as it's fed with sufficient hydraulics.
I'm curious if you can figure out a way to make it work. Please post back your continuing experiences so we may all learn.
If you decide to resell it, send me a Private Msg. Never hurts to have a spare.