The SBO store here has a complete selection of plastic tanks with semi custom options for plumbing locations, standard. Call Dave and he will hook you up.
Advice on access to existing tank and removal requires more info from you. Pics will help as well.
My tank is located under a saloon setee which required cutting the fiberglass under the seat and then re-installing the cutout piece with a wood trim 'ledge', screws and sealant.
It was straightforward once I found the existing plumbing attachment locations.