There are two places you can go buy a motor mount that will fit the Seahawk (or any inflatable that has motor mounts)
1) Army & Navy at Downtown Hastings, their price is $69.99
or
2) Army & Navy Surplus at Broadway Between Commercial and Clark, their price is $84.99
People don't know about Army & Navy (Downtown), but they have a great selection of fishing gear and at great pricing (The original discount store). I go there to get my fishing supplies.
I bought a Seahawk Sport (Canadian version of the 400) back in 2004 and was looking for a motor mount for it for years. but I couldn't bring myself to spend $84.99 for a mount until I went to Army & Navy. I made a self made one but it wasn't very good and was afraid it might rip the slots of the mount off.
They mount works great, I have a 1.2 HP motor and a trolling motor. Pushes the boat great. The mount is for a Seyvor Inflatable, but it works universal with any motor mount with the two holes in the back and any oar mounts that are near the back.
If you want a cheaper price then I would recommend if you had the chance to head to Walmart down the states and get one for $39.99 USD. But they don't usually have it in stock or sell out quite quickly. Walmart Canada doesn't stock it and won't ever. (Try Ebay, sell for $15 ~ $30, but shipping is around $15~$25 not to mention customs, wasn't worth it back in 2004 cause exchange rate was $0.67)
The boat and mount are rated to go up to 3hp. If you going to put that much power, I recommend putting some sort of front bra in the front bottom, it goes very fast and debre can cut the bottom.
I've taken my inflatable up Indian Arm for a day trip. (2 hours there and back). But you can only use Electric Motors on Lakes, cause gas isn't healthy for Lakes.
It's great for Deer Lake, Burnaby Lake, Butzan, etc... (Check the boating restrictions on some lakes. Some allow motors and some don't)