I am also repairing/restoring an old SB45 fitted with a Kirby-Tecumseh engine, though mine is a 14" cut.

The two bolts that you indicate are both 3/4" right-hand thread. As Darryl suggested, I left the chains on, clamped the drive shaft and after liberal amounts of WD40, off they came.

I'm slowly being convinced to replace the Kirby engine with something that I can get spare parts for. I have another cylinder mower fitted with a B&S 2HP engine that I've just rebuilt and it runs very sweetly, so I might substitute that one and put the Kirby in storage.

Chris

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Shed troglodyte

2HP Kirby-Tecumseh Scott Bonnar 45 (14" cylinder)
2HP Briggs & Stratton Morrison 350BD (14" cylinder)
3HP Briggs & Stratton Rover (17" rotary)