Some general rules that might apply here.
If you own an opposed twin, try your best to NOT mow side hills. Lots of engines have died early deaths because of this. Some do better than others.
Change the hydrostat oil and filter religiously every year. In fact, I'd change it right now if it sat any time at all in a location where there are temperature swings.
Pay attention to the service intervals on that old engine, and follow them to the letter. I'd even change the oil early, ahead of the recommended hour interval..
Keep at least one extra set of points and a spare condenser around at all times.
Keep your points file in a handy spot as well.
If it's got a hydro oil temp warning gauge and it turns on, let the tractor cool off for at least an hour.
Don't run the engine at low RPM to moderate your speed. This doesn't let the hydro pump maintain the proper psi.
If it's anything like the old Onan opposed twins, it may benefit quite a bit from pulling the heads and do a de-carbon job on the heads and pistons, and what you can reach of the valves. It only costs you the price of 2 head gaskets if you can still find them.
These are just things I've learned over the years of owning opposed twins and other hydro mowers and farm equipment.