The growth is the rootstock so that is a good sign because its not totally dead. Now as far as the scion(fukushu portion) that I cannot speak for. A picture would be great. Are there any leaves on the top part? You can do a scratch test of the trunk. take your fingernail and scrape a small portion of the bark What you are looking for is the cambium layer to be nice and green and moist. If it is you may have a chance to save it. If not its most likely a goner. Either way unless you want to grow a P.trifoliate you need to remove the suckers below the graft.
mike

I'm with Mike on this...

I'd get rid of those sucker below the graft line as soon as possible...)

Mike

As Mike stated, any shoots emanating from below the graft line, which is clearly visible, are coming from the rootstock. Remove those by rubbing them off at the trunk. You'll know for sure because the leaves will be trifoliate (leaves of three). Anything that is emanating from above the graft line would be from the kumquat scion which we can't see clearly from your photo. Give it a chance to recover. I see the scion does appear to be green, so that's a good sign. Let it have some sunshine outside, filtered light if your sun is really strong in the afternoon. Keep the soil moist but not overly wet, especially at the bottom of the pot. Fertilize with each watering, and I would use 1/2 strength Foliage Pro. It may come back. Watch for scale to reappear and treat accordingly.

Patty S.