Thanks for your responses so far. I think we are getting closer and my foggy old brain is starting to see a glimmer on the horizon.
Let's try this with a bit more info that I realized I did not include earlier.
I can use the usual stuff to set up the partition with Boot Camp Assistant and then I can reinstalll Win7 from the original disk.
The bigger challenge is getting the programs I need from the external Boot Camp drive to the new internal partition.
I believe copying complete apps on messydos is not as straightforward as on Mac and that of course concerns me, The only reason I am going through this hassle is to install a whole whack of flight simulator software, aircraft, and more from years of accumulation and expense.
So if I am staring to think clearly here, the real challenge is getting all the programs copied over in a way that will allow them to be used.
Is there such a thing as a third party program to do that? I know Super Duper can allow you to select specific files and folders for copy buy I have no ideas where all the silly bits for messy dos apps are stored there than in a whole bunch of places?