Ow, that's a tough one. In order to pull this off you'll have to:

1. rip out the motor and controls you so lovingly installed, probably did a real nice job
2. get some coin for the old hull -- that might take 6 months, or maybe forever, since it is a no-motor trihull after all
3. find an Al hull reasonably priced, no motor -- that might take another 6 months, and you're boatless during that time
4. drop more coin on the conversion to pump, I'm sure that won't cost more than a couple grand. Yeah, the guys at 3 Rivers will do a great job, but they are a business after all, and the more monkeying around you do, the more money comes their way.

Honestly, I think you've got a long uphill battle ahead of you going this route. If it were me, I'd fix up the trihull real nice (have it detailed, make it look all shiny) and sell it complete, then go buy the boat (with motor) that you really want. You may do real well if you start listing it in February/March, after the boat shows, where folks see that an 18' Trophy with a 90 HP Merc will cost them $20k.

Regards.

Finegrain
Woodinville