DON - The 10 HP pan will probably bolt up to the 14 HP block if they're all CC spec engine parts but the engine will not run. The 10 HP pan should be a flat bottom pan and the oil flinging finger on the rod cap will hit the bottom of the 10 HP pan. You can cut the finger off but it would be really short and I'm not sure how that would work. The 12, 14, & 16 HP engines have 3-1/4" stroke, and the deep sump pans, the 10's only 2-7/8" stroke and a flat bottom pan.
If the tractor chassis is a NF, the 14 HP engine is too wide across the blower housing to fit in the NF. You'll either have to flare the frame like a 147 or install a smaller dia. flywheel, blower backing plate and cooling shroud from a 10 or 12 HP engine.
If the tractor is a WF, forget you ever read the above paragraph.
I wouldn't go over-board with the clutch. Stock parts should work fine. If you go to a stiff after-market pressure spring it gets really hard on the throw-out bearing at $50 each, plus makes the clutch so hard to push in. MWSC makes a piece to weld onto the clutch T/O lever but that throws the geometry of the clutch linkage out out of wack for disengaging the clutch and engaging the brake. For a working tractor good flat pressure plates, stock clutch spring & friction disk work fine even with lugged tires and a reasonably heavily weighted tractor.
Have to agree that having a LOT of HP in a GD makes for a great tractor. Swapping the K241 into my 72 28 yrs ago made it a much more useful tractor, and swapping the K321 into it 6-7 yrs ago just made it better. It's now over double it's factory HP, and over doubled it's usefulness.
TOM - Yep, mere fact that the 122's are 46 to 48 yrs old now says a LOT about how they were built. Pretty cheap and easy to keep them running over the long haul too.