Two suggestions, Jimmy.
First, if there is a hexagonal bolt head visible in the hole in the middle of the sprocket, use a socket spanner to remove it (normal right hand thread). You may find a washer underneath it. After removing that washer, you might see the end of a shaft the sprocket rotates on. If all of that happens, you might try just sliding the sprocket outward off that shaft.
Second suggestion: I notice that you live in Perth, the home of Alroh. Alroh products are most common close to home. The reel mowers Alroh currently makes, use a belt drive and are not all that similar to yours, but they've been around a long time, making various models. You might try talking to them to see if they made your mower. If they did, they will still have some parts and information. More importantly, if it is an Alroh you can expect to find it is a considerably better mower than an Imperial, which was no great shakes.
