most if not all jet pumps dont have a really substantial brg on the shaft to support belt drive side loading, this might be a problem.
getting back to the portable welder idea.
look around for a miller portable welder, either an older miller 1E or a 2E
the 1E is usually powered by a 12hp kohler, with a 3.5 kwatt 110 volt capacity with good surge capacity. these units go cheap as most are quite old. If the thing welds at all and burns oil like heck it can be cheaply rebuilt using a new rod, piston and rings. I have bought a couple of these for around 200 to 400 bucks. They produce ac power at 1800 rpm and weld at 3600 rpm, welding current is around 180 amps ac which is not bad. Also they are 12 volt electric start.
The 2E series usually have around 16 hp tecumseh engines, and will produce around 5 kwatt ac, some of which were 1800rpm units, so check on this, most also are 225 amp ac machines although there are variants that are ac/dc units. they can be had for around 500 bucks, they are also electric start.
miller also makes newer units, but check to see if they generate ac at 1800 rpm (preferred) and are powered by 2 cylinder onan's. and often are available for around 500 to 750 bucks. and of course are electric start and generally ac/dc welders.
most of the lincoln welders are 3600 rpm units, some briggs and straton powered and other onan powered. they are electric start and good machines but run at 3600 rpm welding and producing power. prices on these are in the 500 to 750 range also.
of course all of the examples above are used units.
as far as i am concerned they provide a huge bang for the buck and are very well designed and built. Far better than any box store unit at twice the price.
i have a 40 year old 1E i bought 20 years ago, and still use it to weld with. works flawlessly and at 1800 rpm for power is a real workhorse. i think i paid around 300 bucks for it, and installed the rod, piston and rings, honed the cylider and ran every since.
just my opinion
bob g