wire gauge and hp of electrical motor
There is some table, you appear to have found it, (though I don't know if you've found a "2-wire" or "3-wire" table - these days, "2-Wire" is a win, and black tape does not belong anywhere in a well, IMHO.)

...and you want our blessing it ignore it? If the pump "lost pressure" possibly from rust, not being stainless, evidently, possibly from ingesting black tape, possibly from 15+ years of wear - I tend to think in terms of "around 10 years, be ready to replace a pump" - the solution is to fix or replace it - adding a larger motor won't help, and adding a larger motor on undersized wire definitely won't help. If 3/4 hp got you adequate flow for 15 years, it will do so in the future, too - more horsepower is not always better, and often comes with a pump curve that may not work at your depth.

With a deep well and a relatively short topside run that's only slightly undersized, you can probably get acceptable voltage drop by leaving the buried wire alone and upgrading to 10 gauge for the longer downhole run in the well.

As to whether the problem is the pump (likely) or the well (possible) it's very hard to tell from afar - you either drop in a new pump and see what happens, or you hire a well company to come and evaluate the state of the well. There's not a lot that a homeowner can afford to do to evaluate a deep well OTHER than dropping in a new pump, and you can screw one up pretty good if you (say) try to evaluate the water level and end up dropping something down the well-hole. How far up the pipe was wet when the pump was pulled?