I have a kubota 6100 2wd that the previous owner upgraded to a 21 horse engine.
I would be a little concerned about putting too much stress on your drive train with that 21 hp engine, since that drive train was originally intended for a 14hp (12hp pto) engine.
TractorData.com Kubota B6100 tractor information
Also, you mentioned in another thread that you were experiencing a loud noise in your 3pt hydraulic system.
If you use a middle buster or subsoiler, you will no doubt be having to go around each stump to break the roots, and constantly raising & lowering the implement. Thus putting more stress on a system that may be hurting, already.
Unless you can afford to hire someone to take the stumps out, in your situation I think you would be better off doing as Baby Grand said in just mowing off the new tender growth that will come up from the stumps until it eventually dies and rots. Or you could simply spray brush killer on the stumps so they will die sooner.