Wayne,
even worse than 1/4-28 threads are 12-28 threads which i have run across on several engines. I suppose this comes from builders who did not have access to the model engineering thread taps and dies being either 1/4-40 or 1/4-32. A 12-28 bolt is .216 o.d., only slightly smaller than a 1/4" bolt. If your threads really are 1/4-28, a standard 1/4" fine thread bolt could be used as a plug or adapted to other threads. I'm not sure what kind of fittings you need or what kind of tools you have available, but 1/4 -28 taps and dies should be available locally and combo tap and die sets for model engineering threads are available on ebay. Once you have some adapters made to go from 1/4-28 to 1/4 40, you can find most anything you need readily available.
If this does not help, more specific details as to exactly what kind of adapters you need would be needed.
crazydoug