Here are some thoughts from me who has refurbished an old 90's L35, vintage motorcycles and other equipment:
My bottom line to you is that while your concern is valid - it is not an insurmountable concern.
You can get common parts like filters and injectors and such for a very long time.
What can't be found OEM will be found in myriads of aftermarket and used parts.
The trick is learning where all the parts places are around the country. The more you look the more you find. The internet searches and word of mouth from parts places and dealers are golden.
I've found in both motorcycle and tractor and other equipment that parts are interchangeable across brands too. An example is my lawn aerator I am refurbishing is a Bluebird, but an identical unit is Husqvarna and that is much easier for finding parts. Sometimes parts are even the same numbers!
An engine in one Kubota tractor might have identical parts in another Kubota tractor even though they aren't the same engine.
Heck, Honda uses bolts, grommets, and parts from some of their cars in their motorcycles.
Sometimes places that rebuild things, like pumps and cylinders have what you need and sometimes it worth "having them do it" if there's something broken (needing parts). If you have equipment like a welder or a lathe, you can sometimes make your own parts or repair what's broken.
I like the idea of having spares around, but you may not be able (or want) to have two of everything.
And then how often you use the tractor or piece of equipment makes a difference on component life. Some folks consider things like hydraulic hoses as "consumables" like filters or oil for that matter.
Here are some places I've used. Maybe others can also chime in on what they've used and done.
Tractor & Equipment Parts, Sales & Service Since 1952. New Holland, Case IH, Kubota & More
www.messicks.com
All States Ag Parts is the leading supplier of tractor, combine, and skid steer parts to North America. Shop for new aftermarket, remanufactured, and used tractor parts.
www.tractorpartsasap.com
Warehouse distributor for many top name manufacturers. Parker Hydraulics, Hypro Pumps, Prince Hydraulics. Direct from the factory. Surplus equipment. We buy large quantities of manufacturer's surplus or excess stock.
www.surpluscenter.com
Ebay and Amazon - Non OEM is not always a bad thing.
Clearwater Hydraulics & Drive Shaft Services
6593 Sidney Cottrell Rd, Oxford, NC 27565