The stock 525 chain is not as common as the 520 chain and therefore has less gearing options available. Still, a quality 525 chain has thicker side plates, higher tensile strength and better wear resistance. Staying with a steel rear sprocket is also the most economical decision as well. Aluminum rear sprockets wear out quickly taking your chain along with it! The weight saving aspect of an aluminum rear sprocket is completely moot on a 325lb/148kg+ motorcycle especially when considering it leads to a much less reliable drive train. If you can find the gearing that will work for you in the 525 size you may just want to stick with it. A rear 43 tooth sprocket from an XF650 Freewind is a common alternative.
Links That Can Help Edit
- Kientech has options for 525 and 520 Jesse also has a steel 45 tooth 525 rear sprocket available. Tange and chain adjustment.
- JT makes some quality steel sprockets but gearing options are not quite as broad.