Put in a 29mm mount. You can have BMS make the rings for you, or you can order an extra set of BT-50 / ST-16 rings and modify them.

As you suggest, if you want to fly with 24mm motors you can alway use an adaptor.

An E30-4, or equivalent reload (E28?) would make for a nice first flight. An E18 might do the job if it is not too windy.