The original sump plug is M14 x 1.5mm pitch.
The kit you show at M15 x1.5 would need a tapping hole size of 13.70mm dia.
Since the hole in the sump left by the stripped thread will already be slightly larger than that, at about 14mm, you would end up with a weak thread.
I don't think there is enough metal there to go to M16 and still have a decent sealing face. My suggestions in order of preference are :-
1)
Do has dogruff says and replace the sump. I think you can take the sump of with the engine in place. An advert for one in the for sale and wanted section on here may turn one up sharpish. You will probably need a new gasket.
2)
Fit an M14 Helicoil type insert. M14 x 1.5 is a non preferred size in engineering but you could use the very common spark plug size of M14 x 1.25 pitch. You would need a new drain plug or make one from a spark plug. You would still need to take the sump off as the frame tube stops you doing any drilling or tapping, plus you don't want any swarf in there anyway.