On the bench with the engine mount back plate fitted....

I like this mount as you can mount it centrally then slide the engine horizontally in adjustment by about 3 or 4 mm. This may sound weak but it comes with quite oversize bolts (hex drive) and decent blind nuts so take it from me it's not coming undone in a rush.... a bit of locktight also helps... You can see I've got a cutout at the bottom of the mount. This is to ensure that the engine can be mounted as far back as possible and is there to give some space for a couple of bolts that protrude from the carb. I've removed the choke mechanism, it saved me 5mm.

The logic in doing this is due to the length difference of the 120 over a standard 90. I have heard that the YS 90 is a long motor anyway with the pump and all so by doing this mod hopefully it will keep the latitudinal response and cg mostly where the factory says.