The monster beetle is a marder with the bigger tires and the bug body.

The off-road beetle is a marder wtih the bug body and the undertray. The body is a different version of the bug and will not work with the monster tires unless you cut off the fenders.

If you want to convert the beetle, you will have to remove the undertray get a new body and wheels. Probably will need different body posts also. May be cheaper for you to just get the Monster Beetle up front.