why change out to a saito 56 when the comp engine in a 4st is about a 70, the nextar is on the edge of its power needs with the OS46 2stroke.

as for the mount any plastic mount will work well enough when I installed a OS61 in my Nextar I needed to use a larger great planes mount, it fit just fine, I did need to mod the fuse to clear the needle valve,and muffler, you not going to need that with a 4stroke,I had the fuse stripped of covering so it wasn't a big deal.

your probably going to need to redrill the mounting holes if you chose a different mount the iso-smooth mount that comes with the nextar is a bit narrow for most engines, fits the 46 perfect, but was way small for my 61.

as for the CG there is going to be only a small adjustment for it you might need to move the battery back a bit,when I added floats and a larger engine there wasn't any change at all.