The YS 63 is only 1.375" wide where the crank fits between the beams. There doesn't appear to be a Dave Brown mount that's right for the YS 63. The 4650 and 5060 mounts are too short. The larger mounts have too much space between the beams, which doesn't allow room for the mounting holes.

The closest fit is the 404L, but is is about .060 too narrow between the beams. The YS has flat areas on the sides of the crankcase where it fits between the beams, so you can't just bevel the corner of the beams a little bit. It would require quite a bit of material removal to fit the engine on this mount, but is probably an option.