To replace the main seal on a Yanmar
GM series engine
Disconnect the transmission
from the engine (you'll need about three inches of space to slide it back and lift
it aside. You'll need to disconnect the prop shaft and slide it back too.
You'll need to brace the rear of the engine or support it from the block and tacke, before removing the transmission
Remove the damper plate and the the HEAVY flywheel. You'll either need to remove the starter and have a helper use a big flat blade screwdriver to keep the flywheel from moving while you break the bolts loose. They are torqued to 60-70 foot pounds. Alternately if you have a 36mm socket your helper can hold the nut on the crank pulley while you break free the bolts. The crank pulley nut is torqued to a higher value (75 foot pounds I think).
Then just pry the old seal out and install the new seal.
Then just put it all back together. Probably will need to check prop alignment too. Easy pezy.