Peter,
I don't really understand what you mean by the weight of the

airframe won't

handle the weight of the single rotary?
The weight of the Motor is usually determined by the W&B of the

aircraft,

this can be adjusted in a number of ways. Putting some counter

weight in the

tail is not out of the question.
If your concerned about the weight being too much for the

firewall and

engine frame - make them stronger.

This whole process shouldn't cost a great deal of money, but

maybe a little

effort!
George ( down under)

Hello George.
Yes, I agree you can mount any weight engine you want on an
airframe and make it flyable but without some limits you may very
well end up with totally different and unacceptable flying
characteristics.  The S-7 was originally designed for a 1 lb/hp
582. With that engine and an empty weight of under 600lbs it was
a marvelously agile machine.. Going to a 2 lb/hp 912 does provide
some climb benefits but there is a noticable loss in agility
which I am very sensitive to. So I don't want to go to too heavy
an engine.
Peter