Tricky, any chemical that will remove the lining will also attack the tank. Wall art may be the only solution.

I had it suggested to me that putting in broken windscreen pieces in the tank wrapping it in bubble wrap and putting it in, or fixing it to, a cement mixer or tumble drier (on cold!) can work well .... but I’ve never tried it! 

I always empty my glassfibre tanks after use.

Dan

Dan mentions the cement mixer trick.

That worked for me with a Honda CB750 tank.

I made up a wooden frame and secured the tank to it, then fixed it to the mouth of the cement mixer drum.

Then put some sharp gravel into the tank and left it for bit before changing the gravel again (as it looses its sharpness after a while on the mixer). 

Then hoovered the tank out with a extra bit of flexible tubing adapted to fit through the tank filler cap. 

Good result in the end.

www.tankcareproducts.co.uk 

The address above is the business of a Mr Ian Potter mob 

07834415921, if there is a chemical solution to your tank there's a very good chance Ian will know about it or be able to give you advice what your options are.