There are many posts on tuning your engine but if you can find an experienced modeler to show you one time I think you would be a lot better off.
On the sizes of the LA and FX/AX you've been getting some wrong info. The FX and AX as well as the FXI and Thunder Tiger will interchange as far as engine mounts go. The LA is an entirely different case [bushings not ballbearings]. The 50SX will fit with a little work on the top edge of the mount but it is discontinued. The 55AX is an even bigger case with a belt around the lower outside of the case.
Fred