RE: st90-k part
A backplate would be tapped to provide pressure to a pump or a regulated pressure system. Without a regulator, you don't want to use crankcase pressure unless you're running at 100% all the time, with little throttling.
As far as the G-61 rear cover fitting the S-61, we don't know at this time. It's been so long that we don't remember, our parts books don't show a cross-reference, and we don't have any engines/parts to do any trial fitting. All we can say is, "maybe", but we can't give you a definitive answer.