Almost anything can be achieved if you have the time, resources and money to throw at it.
Without any one of those three, it could become a difficult proposition.
Based on full time costs, a designer will charge you a few hundred quid at least to draw something up that will work, machining will cost you up to around £60 per hour to make it. Say 10 to 15 hours, if they are a good machinist. The easy bit is you supplying the cash.
The other way is having good friends with the skills and resources to help you out for the cost of a pint or a packet of fags.
I have the plans for a basic fwd/rev crash gearbox if that helps, but unless you can cut your own gears, it will get rather expensive. At least £50 for the gears, maybe more.
Still want to reverse your boat?
Bogs