Except for the power sailors whose transom is built to power boat standards, the max hp for all sailboats is 10 hp per industry standards. However, there are variables in the equation.
Two stroke engines require an oil/gas mixture but due to EPA rules, they no longer could be built starting in 2006. They generally were lighter than the four stroke engines which most are today. It is gas only but for the record, buy ethanol free gas because the corn syrup or corn crap as I call it in ethanol gas., will clog your carburetor. Tohatsu builds their own small engines but also the small Mercury and Nissan engines as well.
I believe te 9.8 Tohatsu was a two stroke engine.
The size engine will be determined for inland lakes, ocean, bay and electric start.
@patbratton owns a Hunter 240 and down sized to a 5 hp on an inland lake which is ideal. l am not sure if a small charging is available for that engine but some folks install shore power for reasons to include a charging system on the boat. I was a former dealer but I am not sure if a small charging system is offered.
@rgranger downsized from a four stoke 9,9 hp Honda that I believe my dealership installed at the request of the owner against my recommendations as I sold the boat originally . Rob down graded to a Tohatsu 6 pro long shaft or 25 inches long. 20 inch is minimal ! If you want an electric start engine, you will have to go for a 8 hp engine due to the size cable but be aware that the Tohatsu, Mercury and Nissan engine for the 8 and 9.9 are the same engine but the carburation is different as the weight will be the same.
For trailering the motor, some sailors store the engine forward or back of the tow vehicle. However if you leave the motor on the outboard motor bracket during transport, you will need to distribute the load of the bolts with larger backing plates versus the washers. In addition, you will need to tie the motor handle of the engine upward to the base of the stern rail seat to keep the motor from bouncing which could cause damage to the transom..
to get a fuller understanding what you want the engine to do for you, can you advise if trailering alot, where you plan to sail, electric start and so forth.
EVeryone has given you some good advice already.
Rick, is that possum alive drinking some grog?
Crazy dave Condon