Well, to put a single engine in a twin frame all you need to do is change the front engine plates (using swingarm single plates). It would seem sensible therefore that to go the other way you would need to do the same (meaning change the front engine plates). I can't believe that you would need to modify the frame - they are not that different.
On a slightly related point - Justin Baker has a Huntmaster engine in a rigid frame, presumably single in origin. Might be worth asking him.
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