A picture might help...I work on outboards a lot, and I am trying to picture in my head how/why it could be seized? from corrosion, or did something happen (damage)?
If it's corroded then maybe a LOT of patience, some PB blaster or Deep Creep, as well as maybe heating it, and working it slowly.
If it's straight out damaged and un-serviceable Dave_E is right cost will scare you from the dealer
and I am sure you have already tried things like ebay without success
Might be best to cash it in and get another motor...but post a pic or more info on how why it's "stuck"
Depending on the damage a machine shop might be able to bore it out, and you could purchase another shift rod...but again depends on the damage