What is a 'Sportscraft Sensation"? Someone might know, but length overall, weight/displacement, some photos would help.
Second-hand sails can be bought on Sail Xchange:
www.sailexchange.com.au/collections/booms-used?utm_source=seabreeze.com.au and Ebay, if you've measured the mast, foot etc.
If you have a choice of motors, bigger is better, providing you can physically lift it. E.g. if you have a choice of between 4, 5 and 6 HP, then 6 would keep you out of trouble better.
TrailerSailorPlace forum will be able to help you also:
trailersailerplace.com.au/phpBB3/search.php?search_id=active_topics