These guys make an aluminum adapter that will give the same mounting pattern as the rimfire cross mount.
I'm planning on using Hacker A20-22L's, which are about 3mm longer in the length measurement than the 30mm that the Rimfire motor drawing shows. The Hackers only cost about $5 more than the Rimfires. I'm sure there are cheaper motors also per the links posted above.
I received my Hackers today and found that they had changed the design since I last bought one. As a result the distance from the back of the motor to the face of the prop adapter is now about 38.5mm vs. the 30mm of the rimfire. I'm sure the motors would still work but from a looks standpoint might stick out more than I care for. Kit is due in on Wednesday, so I'll have to see what it all looks like. I can always use the Hackers somewhere else if needs be.