Removal of the saildrive in April 2011 revealed an almost circular mounting ring, which actually includes the flange/protrusion seen from beneath. One problem I encountered in spec'ing an engine was the length of the outdrive. It needed to be short enough so that the prop didn't fall beneath the keel line but long enough to keep the prop clear of the hull and protrusion, hence avoiding cavitation issues.
After the engine was removed, the compartment was scrubbed and degreased, and the no longer needed exhaust system was removed. I may opt to repaint after the new engine has been installed--since there will be glasswork involved, an unpainted surface will be necessary.
Engine compartment prior to removal
Compartment with engine removed
Compartment degreased and cleaned