It drops straight in the GSX-R frame, with GSX-R mounts.
Only problem might be the sump pan depending on which exhaust you will use. But the sump pan is easily swapped.
I know exactly what's wrong with it: it's f*cked. Time for an 1100 engine.
Seriously though, as Ant says just open it up and look inside. The deeper you have to go the more expensive it gets, and the more sensible another engine becomes.
GSX750 ES/EF/E carrier part no. is 46310-31300. I googled it and came out as 750 only.
https://www.motorcyclespareparts.eu/suzuki-parts/motorcycles/gsx750esefe-1985/seat-613939.aspx