anon125, there are a lot of posts in the archives on the easy way to remove the clevis pin. In my opinion the easiest, because if you do it this way, it makes it easier to replace the motor, is to remove the entire motor and gearbox assembly and then remove the cotter key and clevis pin. There are only four 1/4" - 20 nuts and lock washers involved. First thing to do is to strap or clamp the steps if in the retracted position, disconnect the two wire motor connector then remove the nuts and washers and the entire assemble will be hanging by the linkage and the pin removal then becomes very easy.
Move the entire assembly to you work bench and then replace motor and reinstall.
The cheapest place to purchase the motor is from the maker, AM Equipment in Oregon. Look at past posts for exact name and telephone number.