Oh i dont mind that there is a gear diff for the MI4 also available from specR.

Maybe you can use the hole Gear diff from the MI4. If it has the same width and you can install the Tamiya outdrives....
Mix up with the Tamiya version is not possible because the newer versions like MI4, TCX have a extra grove i think because for a better sealing of the diff.
But the costs of the 37T oneway pulley are about 5$, so it should be no big money issue.

No that is an original Tamiya MSXX MRE 07 lower deck. I only cutted it out like the new upcoming 417x is styled. Should generate more flex. But 1st of all it looks good.

The holes for the split blocks are already drilled in the rear, and in the front i made it on my own to.

The part number for the belts are:

Rear: Xray 3mm x 183mm belt 3054444
Front: HB Cyclone 169T Belt HB70738

This belts are only required if you use the Tamiya 416 18T Center Pulleys.
Of course the newer Tamiya 417 Gear Diff Unit Ver.2 should alsi fit the car.

Greetings Rainer