Delete all of the SPRITES that are not of the BLOKS pre-made body type that you want. Redesign your chosen body type, so it can get added as a character to play as in BLOKS. Then, make it go to 0x and 0y.
This was inspired by one of @TheNotScrub's Roleplays in real life, where you can be any character. The design is based off Lego's Brickheads. Specifically, @The_Great_sansberry's Jack Sparrow one.