I was not able to find the part number for the MKII encoder after much searching and asking. You could always repelace it with an MKIIa encoder (which is not easy). That is:

ALPS Encoders 11mm 15 RES 30 DTENT 0.5mm travel part # EC11E15244C0

My notes show the MKII part number to be:
according to stu:
"It's an Alps product. It is probably part number EC11B15243BB or perhaps, EC11B15243BC. Good luck finding them. It won't be easy."
and Hugo:
"MkIIa is Alps, MkII is a Meggitt part available from Farnell."

But I could never find one to buy online.