Very old Howard 300 rotovator?

I have a very old (possibly 1950s) Howard 300 rotovator with an old B&S engine - engine is now working fine but the chain needs tensioning and there doesn't appear to be a tensioner/tensioner screw. I thought about taking the casing apart but it's not obvious how to remove the gearbox. Anyone know how to remove the gearbox and split the casing? Any help gratefully received.

Thanks