Hi Sam
Pondering this, what I'd probably do if it were my problem to fix is this:
Buy a replacement stay, with the proper mast fitting installed, but without a bottom fitting installed. Get it maybe a foot longer than you think you need. Obtain the lower fittings you need, get compression type fittings (e.g. Norseman). Install the new stay to the mast, and then step the mast, using (as has already been suggested) the halyard to hold things up while you mess about with it. Cut the bottom of the stay to length, and install the compression fitting, fasten it all together, and go sailing!!!
Ballpark, to figure out the length of stay you need, you could measure the sail and add a couple of feet.
- Graham