Tin?? Are you sure??? Itwas discontunued 15years or more ago.
Is it still working???
OK, I suggest you get a high pressure water
blast. Although you haven't said what the boat material is, I presume it is glass.
DON'T SAND!! It's really nasty stuff. I suggest you get an expert to remove it if you are going to go the sanding route
.You need some good protective gear
You should be able to go on top with a standard modern day anti-foul. Copper and Tin shouln't react and if it is really old, there is probably little active tin left anyway. As long as the old stuff is adhearing well, then there will be no issue with the new anti-foul adhearing.