depending on the size.... if you want a motor mount to change from standard length 24mm to 18mm, buy the EMK-18-50 from FlisKits. It's an engine mount kit that doubles as a converter/removeable engine mount...
Jim, I was thinking more in the sense of a Fat Boy or something with a BT 100, having a 29mm mount, interchangable with a 24mm cluster arrangment. Although the more I think about it, the less practical it seems.
You could start with our EMK3-24-70 which is a 3 motor 24mm cluster that comes in a BT69 which just fits inside a BT70 tube. that would allow you to make the removable 24mm cluster mount. Then you could buy a pack of BT-69 tubes to scratch the 29mm alternate mount.
Note that the BT-69 conversion is one of the suggestions for the Rickter Recker, since it uses a BT-69 cluster inside a BT-70. Maybe a 29mm option for the RR?
Another option is to build a series of different mount configurations inside tube couplers. I've got several tube couplers that all fit inside a BT-80, with everything from a single 29mm to 4x24 to 1x24 to 4x18. They're all designed to fit my Estes Phoenix, which I friction fit them into out on the range when I can see what I want to fly that day.
do a serach here in TRF this was discussed maybe a few months ago and I attached a drawing showing how you could create a qucik and dirty removable motor mount system......