Oh, I've missed the prompt welcome :) So, hi there buddy!
Starter mount, as far as I know, is a part of transmission case bell housing. Separating the bell housing from an engine is a PITA, as some of the bolts are well hidden (9 in total, as some of the diagrams indicate 8). I would rather think of resizing the threads, making an inserts or welding the part that broke off.

If You like the Blazer and want to restore it, my advice - do not hesitate and make a 2" body lift. It gives pretty good access to lots of the spaces (like starter, plugs and so on, including bell housing bolts).

I am not 100% sure. but I guess that ZR2 has a suspension lift not the body - RC says that their suspension lift will not give 2" for ZR2 as it already comes with a lift. The body lift extends the rubber cushions between the frame and a body for 2", so if Your cushions are shorter than that, then it's not a body lift.

Good luck with Your swap. While doing mine, I have realized that lifting the body is an easy way of separating the engine and a bell housing and I've left it lifted.
Pay attention to the transfer case cooling lines and the engine oil that are clamped to the oil pan.