Here is what I would do, as I have done a number of times in the past.
Find on the Internet
, or from a Nanni dealer, a brochure on the model you have installed now. This should include top, side, and front drawings with measurements of engine.
With 3 or 4mm plywood
, make a template of your engine bed
With the side view, draw and cutout engine shape above and below engine mount line, or sometimes crankshaft centerline, and tape or glue this to the fore and aft centerline of the bolt layout template.
This will give you basic 3 dimensional of engine.
On the templates you can make notes of parts
where they stick out from engine, to clear other things, such as batteries, thru-hulls etc.
Do the same thing with the engine you would like to use, and compare sizes.
You should get a good idea of what will fit
For height determination, look carefully at relationship between crankshaft, gearbox
output (propellor shaft) centerline, and where mounts rest on engine beds.
Also look at exhaust
outlet, and waterpump intake location on the different engines.