I just received my RF unit this week. I went with the 5-hole model (#20310). I believe this is the same (except for the finish) as Summit's PRF-20308.
The unit is 3-1/2" in diameter and about 5/8" thick. It has a flat back that does not protrude into the wheel opening.
As you can see from the photos, it does cover the mounting holes on the Moto-Lita. The holes, of course, don't match.
It would be possible to drill the top 3 (to match the RF unit) through the Moto-Lita and tap them into the FFR-supplied hub. The bottom 2 holes don't line up with (but do conflict with) the existing holes, so you may not be able to use them. You could always glue in a screw head (for looks) if you don't mind you wheel held on with 3 of the 5 screws.
Painless also makes a version without the holes:
Painless RF Center
It may be possible to simply attach one of these over the Moto-Lita's holes with double-stick tape or silicone (you can't make it permanent so that you can change the battery).
What circuits were you planning on running with the unit?