The YSM 12 is a big single cylinder, slow reving engine. It produces a LOT of torque. If you compare the specs, the YSM peaks between 3,000 and 3200, the newer higher hp engines peak at 3600 or higher. I would sail the boat and then decide. Torque moves boats, horsepower sells engines.
I have not seen torque specs on the YSM 12, but seat of pants, it is fairly powerful.
YSM ownership is a love /hate relationship. Reliable, powerful, economical but will rattle any loose fillings out of your mouth. They didn't call them YanHammers for nothing. Still and all, after sailing with Thumper for a while, you develop a lot of respect for it. If its a new to you boat, sail it for a while before you start on figuring spending a lot of money to replace a running motor. I'd consider a 3 bladed prop before I jumped to a re-power. Unless you are planning on keeping the boat for longer than forever, the economics of a re-power are against you.
And few experiences equal the transition from power to sail when you shut down a YSM 12!