I finally received a new motor for the UMX F4U via eBay, it took a little while.
The RX/ESC/servo board had to be lifted up from its seat to make room for the motor as it was pushed back out from the gearbox, but that was easy. There was just a tiny string of glue on either side along the board and it lifted up easily with minimumum damage to the foam seat.
God thing I didn't have to remove entire gear box - that would have been a lot more difficult, with damage the foam seating.
The new motor revs as it should and spins the prop with loads of power, so we're back in business...Nice!

One thing I noticed with the hot glue - the nozzle tends to leave a thin trail of glue, like a thread as it is lifted away from the puddle that you deposit. One such thread landed on a servo output gear and messed up the gear mesh badly. It took quite a bit of work with a loupe and the tip of an scalpel blade to sort that out.
The original glue is white, opaque and very elastic. Does anyone know what it is, and where to get it?