Kz900. With kz1000 engine

I picked up a 76 kz900 with older 1000 engine. Was told engine won’t come out of first. I have a 81 1000 engine at the house. Wondering if it would bolt in

The engine has a very simple positive neutral finder mechanism in the gearbox which won't allow any other gear to be selected except first and neutral at a standstill so that is most likely the (non) issue.
Bike needs to be moving or on mainstand with wheel spinning to select all other gears.
Later non kickstart engines need engine mount mods to fit.
It's not a straight bolt in fit.