Yep, most of the time, companies build stuff using off the shelf parts as much as possible. About the only time they don't is when what they need just does not exist, or they want to be the exclusive supplier.
I needed a drive gear for a winch on my father's truck once.
The motor had developed problems, and I pulled it off to get a new one. Put the mounting bolts and the gear off the shaft in a small box so they would not get lost.
And, of course, the box got thrown away.
The company that I had bought that winch from was out of business. But a friend of mine had bought one too. I went over, pulled the motor off his and got the gear. Went up to Dothan Gears in Dothan Alabama and showed it to them.
They measured the shaft size, diameter, and gear pitch.
Couple of minutes in a catalog, and they had one on the way. Came in about a week later, fit and worked perfectly. Not sure, but I think it was even made by Boston Gear.
Of course, you don't have a gear to go by, but you know the size of the shaft of whatever motor you decide to use, you can measure the gear pitch off the driven gear, and you can measure from the center of the shaft to the inside of the driven gear teeth to determine the size.
It has to be bigger than the motor shaft to fit over it, and has to reach to the driven gear. There will probably be a couple of diameter gears that will fit, just guess at what ratio you want. Probably the slower the better. Less load on the motor, and you don't want the boom swinging too fast.
