Glad to hear that you found a boat, even if it's not the the one with the 90 hp

A man needs a boat. It would be a shame to die with too much money in the bank. You need to enjoy life while you're living, right?
I purchased my gate valve from Blue water Yachts (bwyachts.com).
Whether you purchase the gate valve from bwyachts, an RV supply store, or Highlander, you should be good either way.
Regarding your question about the T-handle going under the motor mount, I doubt that you would have to remove the motor. The T-handle screws onto the top of the rod which is connected to the valve. With the T-handle removed you should be able to slide the rod under the motor mount. It's hard to say for sure without seeing a picture. Post a picture so that we can give you better advice.