The 600w uses the same motor mount spacing as a Kollmorgen 400w.
Tim says they make bigger ones. I'm not sure how available they are in larger sizes. They make a 1.5kw continuous rated one, but the diameter is larger. I'll bet I could get about 10kw peak out of one of those. I have no idea on price. I'll try to find out.
I like the idea of a "standard size" mount that would allow easy swapping.
As it is now, be prepared to drill holes. It would be nice to have "standard" throttle, hall sensor, and brake switch connectors while we're at it. It really wouldn't be too hard in theory, like ISA was to computers.