Did it look like this?
I'm guessing that I would have to buy a gearbox of some sort to use it?
Yes that is similar to same as on my L185. There were versions like your picture show the crankshaft pulley being held on the crankshaft with the large nut. That means that the crankshaft is extended and the splined area is still part of the same crankshaft. The other versions had a crankshaft pulley that was attached with a bolt into the end of the crankshaft. That pulley had tapped holes that held on a short (stub) shaft bolted onto the crankshaft pulley.
You can use a gearbox if you want to, I believe I have seen a small transmission driven off the front of an engine so the output speed is adjustable to some degree. The most difficult part of using the crankshaft is getting a splined part that fits onto the crankshaft splines and getting a good flexing coupling involved so you don’t break the shaft off the engine.
And remember that shaft’s speed is the same as the engin speed and will vary some under load.