Good luck in finding an expert. There weren't that many of the TOCK's built and finding someone who's changed a tank in one is even less likely.
Where is the tank located, what furniture, sole,
engine, etc have to be removed to get at the tanks?? Are you going to be able to remove these impediments without destroying them?? If you have to destroy stuff to get the tank out, what's it going to cost to reconstruct these pieces?? What is holding the tanks in place and will it work with the new tank?? Last but not least, will you have to have a custom tank built or can you buy an off the shelf plastic/metal tank. If an off the shelf tank will fit the space available, will the fill tube, fuel line out, vent line, and guage location, if one is needed, work out for you. These are the questions you or your contractor are going to have to answer. You can probably come up with a pretty good estimate of your own if you do some basic background work.
I just had to remove the tanks in a 30'
race boat. The tanks were under quarter berths each side. Had to cut out the plywood over the tanks with a saber and hand saw. Then break/grind out
fiberglass tabs along three sides of the tanks before I could get the tanks out. Had to glue and screw cleats in to support the no longer monolithic plywood area, and provide material to rescrew the cut out plywood back in place. Took me more than 4 hours to get the tanks out and rebuild the space to put the tanks back in largely because of the inaccessability of one side of the tank. That was the easy part,
money wise. New custom aluminum tanks were $900 for two relatively small tanks. Probably will take more than two hours to reinstall the tanks by the time they are tabbed back in place and the plywood screwed back down. So about 6 hours labor, $40.00 worth of materials and $900 in tanks if you hired someone to do this relatively simple tank R&R.
You are going to have to assess your tank R&R. Have seen jobs that required dismantling about half the salon area, others required
engine removal, very few are easy.