How do make a functional arm using an A-Chassis model?

So I’m thinking of instead of doing games, I’m going to try to do model development, then selling them for robux (attachments would be free) using dev products on a showroom game. Now one thing I’m going to make on Roblox for my first thing, is a Bobcat Toolcat 5600 just like seen below.
Now here’s the thing, I can make the thing drivable using the A-Chassis model, but the arm will be out of use, and I don’t want that to happen or else it would be useless, so I want to make the arm functional by using the number pad or something. and also I want it to not be able to move unless the engine is on just like how it cant move with the A-Chassis unless the engine is on. So how would I make this?

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