Are you going to remove/repair or replace? If replace, do you already have another engine to put in?
What's wrong with the current engine?
I swapped the dead D750 in my
B7100 with a D950 from a marine generator set. I've heard of others using D850's from portable construction site lighting rigs. The bellhousings of the D750, D850 and D950 engines are all the same, and flywheel/clutch match. The advantage of D850's and D950's is they have a real waterpump instead of the percolator cooling system. The oilpan and pickup tube in the gensets are often deeper but can be swapped. Generators also may have different timing covers (i.e. oil filter in the wrong place) and have different governors that are made to hold constant RPM with load changes, which aren't great for tractors (although I'm currently using it). The timing cover can't be swapped from a D750 onto a D950. A remote oil filter kit enables clearing the tractor frame and mounting the filter elsewhere. Waterpumped engines have smaller diameter coolant hoses, so have to be adapted to the large diameter radiator ports (copper plumbing fittings work). Heads from D750's and D850's are the same and/or interchangeable. D950's have larger diameter valves that don't fit in the cylinder bore of a D750, but might fit on a D850. Also, non-tractor motors may not have the splined front PTO shaft on the front of the crank. I fabricated a bolt on shaft stub with a keyway (which is easier to find a coupler for than the odd spline.)
Removal of the engine is pretty straightforward - use an engine hoist, block up the middle of the tractor, remove hood, radiator, front axle subframe, bellhousing bolts and the engine slides forward off the transmission shaft. Re-installation is the reverse, except if you disturb the clutch/pressure plate you'll have to make sure it's it's very well centered or the transmission input shaft won't line up with the pilot bearing in the end of the crank (I'm not sure if the HST has a clutch...) Be careful because the back half of the tractor can tip over backwards pretty easily - blocking under the drawbar helps.
If you have more specific questions, fire away.
