If you've managed to get INTO the body, then you have everything you need to complete the body. Now of course it will be easier if you have all the health upgrades, plenty of food, and a few good pets by your side, but the whale etc. are not "needed" to complete the game. You get a save point and a turtle just before the final boss so don't worry about missing things and not being able to go back. You can go back now if you want, the body will always be there.
Getting all the treasures is completely optional, however, you might want to go and find how to get into the whale because that's needed in order to get the secret ending.
