I have recovered worse calipers than that. Obviously a replacement Fzs1000 or post 2009 Xjr1300 caliper is the easiest option
I use wonder Wheel alloy wheel cleaner to clean up the caliper first.
Use with care as it is acid based, follow the instructions exactly, then wash it down well. Check if you can open the bleed nipples and then remove the pistons if possible. You need to check the seal slots. If the seal slots cannot be cleaned up than replacement is the only solution.
If the nipples are removable and seal slots can be cleaned up; replace pistons and seals and nipples if necessary.
Dismantle the two sides only as a last resort as the small seals can be very difficult to get.