Hi Pete,
The STS range of engines are GREAT !!!!!!
I race a .12 one on a gearbox in a Datline Cheroke chief mk2. This keeps up with the novarossi.
The main UK supplier for STS engines and parts is CLEVELAND MODELS 01642 241511
www.clevelandmodels.co.uk if you req the correct collet he should have one for the .28.
There are two ways of doing your flywheel, in both cases you will have to cut off the plain end of the crank, just in front of the threaded section, using a small disc on a dremel type tool. You are now left with a plain section - then the thread.
Fit the collet and then measure and work out the size of flywheel that is req,d leaving enough thread to take your coupling. :-\
The other way is not to use the collet, but just have a straight line bore through the flywheel and a small thin washer behind, pushing up agaist the inner race of the front bearing. The only problem that does happen using the second method, is the flywheel has a tendancy to come loose as it does not have the collet to keep it tight on the shaft.
I take it that your engine has a pullstart, so no need to make a groove fo a belt. The flywheel does not need to be very big in diameter.
Hope this has given you some ideas? :-\
Gary