Looks like that 15 amp ESC does brushed only.
Roughly call 15A x 8.4v = 126 watts.
Would you want to keep the same batteries or 'upgrade' to lipos?
Does it have a gearbox?
A 380 sized motor on 7.2v I see runs near 20,000 rpm? Anyone have a guess if that's right for this plane?
I'm a bit out of my element, but a quick kva calc, 8 volts, 20,000rpm, 2500kv?
So a 125 watt motor (or a bit less) 2500kv is 'in the ballpark' at a guess.