Little Machine Shop sells a belt drive conversion kit for X2 mills. It's their part number 2560. I looked at the "compatibility" tab, and though the Griz G0516 and G8689 are listed, the G0781 is not. http://littlemachineshop.com/products/product_view.php?ProductID=2560&category=
It would probably be worth your while to give them a call. They're very friendly and helpful. Their phone number is 800-981-9663. Best of luck!
I am having this same problem. I broke all 4 gears on the inside of the mill. The kit linked above looks like it would fit but it is slightly off. The spindle size needed for the kit is 30mm. The spindle size on the G0781 is 28mm. Where the bearing sits is 30, but it necks down to 28.
I did email Chris and he confirmed that this kit will not work. I am considering pressing a sleeve on the spindle to get to the 30mm. Everything else looks like it would fit. The DRO for the spindle speed would probably need to be relocated a little, depending on the pulley size.
I did find a kit from benchtopprecision.com that is currently not in production. Scott from that company said theirs will work tho.
Any luck with yours? It looks like it’s been over a year since this was posted.
Thanks for any information you can provide.