I would be a bit leery of using a hot tub pump to lift water from a well. My experience is with pool pumps, which have to be primed, even when just a little higher than the level of water in the pool. Priming a hot tub pump to get it to lift water from below would be a bit questionable in my mind. They are a bit different(?) from pool pumps in that they are used to circulate and run 'jets'. Pool pumps also have their outlets, which are similar to the jets.
You may want to look at amazon, as I have seen multiple units for less than $200, brand new, that will produce enough to run several pumps at a time. A 3.5hp engine may not produce as much Kw as you need for several pumps. I'd check the tags on the pump motors for their power requirements.
Flathead engines need valve adjustment due to wear of the valve & seat. That wear will move the stem downward, towards the cam lobes, which will decrease clearance.
Proper clearance will be given in a range, such as .005-.009". As the valve wears, the clearance will close, and the only way to increase clearance, and get back into the proper range, is to shorten the valve stem. A valve stem is ground or filed shorter, checking very often as it is 'adjusted', until the clearance is in the suggested range.
Most people will use a bench grinder, applying the end of the stem perpendicular to the face of the wheel. Grind off a bit, put back into the block and measure, tappet sitting on the heel of the cam lobe. More or less. This would be done after re-surfacing (valve grinding)the valve seat and the valve face, or lapping them using compound.
A flathead engine cannot produce as much hp per unit fuel as an ohv, generally. The flow of air through the engine is more restricted in most cases as the passage from the combustion area(over the piston) to the exhaust and intake is more restricted than the more direct passages of an OHV engine. So they use more fuel. The pressures of the EPA, CARB, etc pushed the small engine manufacturers to produce OHV engines for the push/self-propelled mower market, not fuel economy or lack of power. The OHV engines run cleaner as they can clear the combustion area better. I think. Most lawns would be fine with flathead, but external pressure has forced a change. When running a generator, you might be concerned more with fuel usage than when running a mower for an hour or so weekly.
If the engine has good compression, and will re-start readily when hot, it may not have had enough use to need anything done to the valves at all. I would definitely do a test of the pump equipment I was counting on for use during an emergency.