I weld, both metal and plastic.
There are two problems with welding plastic.
1.matching the rod material to the item being welded on. There are a ton of different plastics out there. The best match will be a matching donor item. Anything else is problematic.
2.Age of the tank. Plastic degrades over time. At the 14-15 years of this tank, it is a tossup as to whether it will work or not.
If it were mine, I would give it a shot. If it was a job for a paying customer, I would pass.. Too much risk of it not working , and having to refund the money. I don't work for others for free.
Other than that it is still probably a somewhat difficult repair as it would likely be an overhead repair. Doable, but not easy, especially if the rod material is not perfect