Hi Binbag,
Welcome along!
I too am relatively new to power and have also got some OS15 and 20fps to use in free flight power models. Originally OS did parts to convert the engines for use for free flight and control line. These are the venturi and needle valve assembly. Please see photo attached of a 15 and 20 with the venturi and needle valve. These parts are still available from Tower Hobbies in the USA, I think the venturi is about 7 dollars and the needle valve 12 dollars. Until recently they were listed as still available from Ripmax in the UK, as soon as I inquired in my local model shop they were withdrawn from the Ripmax website!
The venturi is a simple thing to machine in a lathe. The bottom part of it will be the same as the rc carb fixing, only the holes for the retaining screws will be used by the needle valve assembly. The needle valve holds it all in place. For the 15 sized engine the hole in the venture to let the air in should be about 6mm to run on suction (so I'm told) and about 7mm when on pressure. I don't think the actual shape of the venturi is that critical (I've seen lots of variations on different people's models.
I presume it would be possible to use a needle valve from a different source, for instance Nelson ones from Doug Galbreath, but I haven't got around to finding out about that.
I hope this is of some use, I'm sure there are people out there with a LOT more info to impart on this sort of thing, I too will be happy to hear it.
See you on the flying field sometime soon

Adam