If you are just using it to get in an out of harbor, I have a friend with a trifoiler, and from watching him struggle coming in and going out of the harbor. I would think a small lightweight electric would be ideal for a trifoiler. The one I would suggest is the Torqeedo 403, it very light weight (15 lbs including the lithium manganese battery), has really long battery life compared to anything else out there. It is completely water proof (including the battery which floats). and has a remote throttle with gps electronics that calculates range based on power consumption. Pretty nice. The whole thing can be mounted on a ball fitting so there is only one small attachment point to the hull, and can be easily removed when not needed.
It actually designed for kayaks, but I would think it could push a trifoiler around pretty well (several TI users have them, so it will propel a larger boat). I would just keep it locked straight and steer with the trifoiler rudder, alternately you could steer it with a couple strings to the cock pit if your really wanted to. Of course the remote throttle can mount anyplace you like (also has reverse which might be nice if docking).
They are a little pricey (about $1800) but well worth the extra expense, not having to carry 80 lb lead batteries around (that btw I cannot lift, I know I tried with my sister in laws boat, she had an inflatable with two 80 lb batteries and a Minkota 30# trolling motor, I vowed at the time I would never ever attempt anything like that, the experience actually marred me for life ( LOL). Even with all that battery power her and her husband got stuck off shore on lake Erie, miles out, and it took them all night to get back in (literally all night paddling an inflatable, they had to be totally exhausted (could have died actually).
Hope this helps