Hi Ian, If you understand motors then the following will be little help. The original motor was a resilient mount 1/2 Hp motor at 1420 rpm and for most 240V. Ml7R and S7 used a 3/4Hp motor and there is no reason not to use this slightly higher power. Modern motors express the power often in Kw so 0.37 Kw (1/2Hp) or 0.55Kw (.75Hp). I used a solid mount .75Hp on an old S7 and it worked fine, and it was to hand.
If cost is not important then you could upgrade to a VFD conversion of which there are several ways from buying a suitable motor and VFD unit to a pre wired kit you just bolt on.
The motor shaft is 5/8″ from memory. Good luck. Noel/