You can still find first gens with decent engines that you could buy and pull, but those with low miles are few and far between, and if you find such a monster, you might find it easier to just use the whole vehicle instead of pulling the motor and putting it in your '91.
It's also worth noting that ALL OHV Explorer motors have similar issues, so even a lower mileage one will eventually need some stuff to improve these. Most common are worn rocker arms, pushrods, leaky lower intake gaskets, and sometimes even cracked heads and manifolds, especially on '91-92 models.
If you can pinpoint the oil leak on yours (head gasket, valve cover gasket, rear main seal, something else?) you can get a better idea of what you have to look at for repairs in terms of refreshing the current motor over buying something else to drop in.
It also depends on the shape of the current engine and what you know about it. If you bought it new and took care of it for those 196,000 miles, you might be better off throwing a few hundred in parts at it, which might make it last another 200K and beyond.
To give you a general idea of what you'd be looking at for a refresh, you'd spend about $400 on new heads ($200 ea), $200 on new rocker assemblies, $50 on pushrods, and $100-200 on gaskets, and then maybe another $100 for tune up materials like plugs, maybe new wires, etc. and then new hoses and belts. Maybe somewhere around $1000, and you've essentially fixed it up to better than new shape and taken care of the current and most possible future issues. Compare that to a rebuilt engine for $2000+ and it's a pretty good deal, though if you can find a rebuilt motor with a decent warranty for closer to $1000, you might seriously consider it. My experience with most rebuilts is they tend to do the cheesy repairs on wear items like the rockers, rather than replace them, which makes even a rebuilt questionable unless you can find out for sure they use new parts for such things.
It's your ride and it's up to you, so you'll have to decide what you're wanting to do, how much you want to keep it, and what you're willing to spend on it. If you can do the work yourself, just this basic refresh, or even a full rebuild can be very much worth it, if you can afford the down time, or even locate a motor to use in the meantime while you're rebuilding yours, or you could even rebuild another motor and then change it out. Overall I'd say spending $1000 or even more isn't too shabby if you plan on keeping it, at least compared to the expense of buying a whole new vehicle, or even a used one.
