I would first install a concrete anchor, in the tank part you need to cut out. Secure a strong rope to it, and tie other end of rope to a tree, or something else. This is to prevent the piece from falling in tank.
Cut the top out. I would use a concrete saw, and maybe impact masonary drill bit.
Wouldn't want to sledgehammer it.
I just replaced my outlet baffle, and installed one, that had a pullout, washable filter. Will help to keep solids out of leach field.
My tank had no inlet baffle, which caused the inlet pipe to get clogged by the scum layer.
I installed a PVC inlet tee, and down pipe (baffle)