I would ask for the supplier to send the appropriate sail arm servo or approriate substitute sail arm servo. Especially if that is what you ordered and ask them to refund return postage.
This is not a suitable substitution.
You cannot put a sail arm onto a drum winch servo.
The HS 765HB Sail arm servo rotates 140 degrees, just under half a turn.
The sail winch provided rotates 3.5 turns.
If I were you I would stick with the original set up of a sail arm. You shoud not have to compromise because of a supply error.
Demand a replacement or your money back, and don't accept any BS.
If they are still unhelpful, and if you used a credit card, tell the supplier you will contact the CC company about stopping the payment, if you are not satisfied. This usually gets them moving.
But remember to be nice.