Hi!
Never ever add weight!!! That's a death sin!
Move what ever it takes to reach a good balance, servos , batteries receiver, tank you name it ...but never add anything!
Can't you Dremel away on the engine mount. Plastic engine mounts often have long engine bearers that stick out, cut them off. Move the engine back...just remove the firewall and glue it in further back. just some centimetres might be enough.