What is wrong with like a used minn kota or motoguide?
It sounds to me like what your doing would be more work and expensive then to but a used electric motor. They have all kinds of sales on these at Wall-Mart, BPS and Cabelas. Most of the time they are last years models that would be perfect for what you need.
Perhaps someone on CC has a used one for sale.
http://search.ebay.com/search/search...rclo=&saprchi=
http://search.ebay.com/motorguide_W0...pqryZmotoguide