The 923 gearbox is a very rare bit--only 2099 912Es were made, and those were the only cars to use that gearbox. The 923 is a 915 transmission (as used in 1972-1986 911s) with a 901-style input shaft. That way the engine could use the 914's clutch and flywheel setup.
I think that Kennedy (
http://www.kennedyeng.com ) makes a flywheel/clutch setup that will let you mate a Type IV motor to a 915 gearbox. I am not certain of that, though.
Mounting the motor and getting the engine tin fitted correctly will take a little work. Both are very doable, if you can fabricate a bit and weld, but both will require work.
--DD