$800, not a bad spot to be in. So, you don't think the front part of the frame is usable either? I can't figure out what the issue is with that top tube, oversized tube over the stock tube? TC Bros instructions are online and might help to describe where to cut and what stock part of the frame is kept when using their hardtail. If the front of the frame hasn't been altered to "unusable", maybe a gusset or two can save the front of the frame, a few shared measurements could help you decide.
I'm using a recycled TC Bros hardtail and was impressed with the workmanship of it, considering the retail price, except for the rear lower cross member that was tacked in 1/4" out of square to the other tubes, glad I caught that before mounting the battery and electric box. That 1/4" would have been real noticeable with my other parts on.
Scott