Did the timing belt have a tensioner or just have to be rolled on bit at a time?
It's not a timing belt, it is a drive belt. The crank turns the flywheel, it turns the dog that turns the belt. The belt connects to the top gear by a cog twice as large as the lower dog (this gear reduction) That top gear drives the propeller shaft.
To change that belt you have to support the motor with a Jack, remove the rudders, propellers, prop shaft and top gear and the gear reduction housing. Put the belt on and reverse the order putting everything back.
The hardest part is getting the top cog back in under the belt inside that housing. I cut some 1\8 inch metal rods to put through the bolt holes under the belt to keep it pushed out towards the sides to get the cog started under the belt.
The top gear is called eccentric. When you put it in under the belt you rotate clockwise to swing the gear until the belt gets tight then you bolt it up.