I'm in the same boat at the moment, I have a cheap ALDI one, the one where the belt leans back at 45 degrees for some reason that escapes me ?
However 3 or 4 new holes and it's now near vertical, 50mm wide belt but no sanding disk, those have never appealed to me and what is the difference between a disk and belt anyway,? it's just rotating medium.
Anyway after a thunk, which in my book is a serious think, I have come to the conclusion that all these cheaper belt sanders lack power and features, so I hunted round and got a 1.5 HP 3 phase motor as I know these are 1.5 HP continuous. I have 3 phase so no problem but I also have a couple of spare inverters so have the choice of running on single phase if needed.
Idea is to remove the shaft and replace with a longer shaft with provision for mounting wheels both ends.
Left hand end will have a normal course general wheel out board with a 1/4" wide white allox wheel inboard of that one with a tapered shape to it for the odd clearance angle.
Right hand side will carry a wide fine while allox wheel for fine tool grinding and outboard of this a 50mm wide belt sander.
Unfortunately I will have to loose the fan off the motor to do this But TBH I can't see a problem as it will not be in use for long periods of time. If I do go the single phase VFD route I can at least wire the inbuilt thermister to the VFD but I feel it's over kill.
I propose to use 8" wheels on this because of a couple of reasons. 8" is still within safe grinding speeds but most important is that I literally have a shelf full of 8" wheels. Also the flywheel effect of these wheels if correctly balanced will help smooth running.