Ransomes M21 self propelled mower engine swap.... possible?

Hello, new to the forum.
I have a Ransomes M21 self propelled mower - guessing late '80's. The engine needs a lot of work and I am wondering if I can replace it with a modern unit. My issue is that there is a shaft and pulley on the Briggs & Stratton 4HP engine that comes from the side. This pulley operates the self propelled mechanism. Would there be a new engine that would have this fitment to work with the mower and keep the self propelled mechanism working? Thanks in advance.
 
Sorry I  have tried to upload photos but can't seem to do so.