I have a YS 53 four stroke I would like to mount in a VQ P-51B Mustang; has anyoune done this or with similar 4 Stroke without butchering the cowl.

Mount the engine inverted to avoid unnecessary butchering the cowl. All engines, except for the RCV 60SP, will require some cutting of the cowl. You will need to cut an air exit hole at the bottom of the cowl anyway.

I dont think that engine is big enought for the VQ P51 B I think it calls for a 91 4 cycle

It's only a .40 size plane with 58" wingspan. Plus, the engine is a YS, so it puts out almost as much power as a .91 4-cycle.