I also have the MK2 PRO with partial Racy OP. Unfortunately only Racy have the Racing Chassis, but too bad no more in production and I hardly can find this. Is a very nice car with almost fully OP, except some minor part. Engine also not bad. but if you need power, you would want to change it. I am planning to change it to the OS Mspeed but hardly can find this engine.
Its true, you can change MTX3 2speed to put in MK2. I had my MTX3 as well and its interchangeble.