A JP2 was actually my first choice, but demand for those is always very high and they end up being too pricey.
Spot on analysis! In the UK it is the canal boat scene that drives the demand for JP2 engines, and they won't think twice about carving up original JP2 startomatic generating sets, just for the engine.
It is common to see the JP4 engine broke up for spares, particuarly rough examples.
Still at 2k lbs it's all there.
For that reason plus the C-19 situation, it might sell for a lot less than would normally be expected, could be an opportunity to get an all up together engine for peanuts, take the electric start parts from the JP3, and convert the JP4 to electric start, and you have the makings of the ultimate off grid power source
What is parts availability like on the JP type engines?
They do share some parts commonality with the CS range (top end parts): Piston rings, valve guides, springs, fuel injector, fuel pump delivery valve, valve cotters, valve spring retainers, rocker arm striking pad, rocker arm adjusting screw.
Parts availability from India is very good (but you have to know where to look) Dev precision would be a good start, and if you need cylinder heads moonrise industries.
Genuine parts can be found, but are not cheap, genuine piston and liners are becoming challenging to find, but if you ever need pistons I do have a source. Or you could modify an 8/1 piston (bore out the wrist pin hole to 1.5/8"). I know of numerous canal boat engines in the UK have had that modification