IF you're not locked into using the ring gear, and IF you have an extension (>2") on your prop, you can very likely use what is called a steel or aluminum "shaft collar" on the extension to add mass at the front end.
These are available in inch or metric diameters in a wide variety of lengths and thicknesses from a number of suppliers. Prices, depending on material and finish, are $200-ish for inch sizes and about 30%± more for metric sizes.
I currently have a Ruland 135mm metric collar on the hub of my too-light MT prop that adds 7.2# to help with CG concerns.
Andy Elliott, CFI
Flight Reviews, Dynamic Prop Balancing
KFFZ, Mesa, AZ