Since nobody has responded, here are some thoughts. First a caveat, I don't have any FP or LA 15s or a Brodak 15.

Checking the parts lists for the OS engines, they all use M3.5 screws to hold the RC carb so the holes in the crankcases will be clearance on 3.5mm dia. The standard OS NVA for the FP/LA 20–46 is 3.5mm dia so it will fit. Curiously, the FP 15 spraybar measures 0.123" dia so I guess that it is a rattling good fit in the crankcase.

The NVA for the Brodak 25 and 40 is 4mm dia and I have noticed that the NVA listed for the Brodak 15 is different to the 25 and 40 but the web site does not tell us the diameter.

It looks like you will be OK with the OS NVA but if you insist on the locking collet type NVA, you will probably have to drill out your crankcase and venturi. Either that or contact Steve Rothwell in Sydney for his NVA to fit the OS engines.