Bear in mind that possible and practical are not always the same thing.
e.g. My 20HP Yam 2str CAN be upgraded relatively easily to the 25HP which was based on the same design. To do this I would need to replace the carbs and the exhaust manifold. To make use of the extra power I would need a new prop as well - that lot would add up to around £500 assuming I fitted it myself.
Probably easier to sell and buy a replacement.
For someone to be able to confirm if there is a "simple" route to upgrade - they will need to know the model and year.