Whelp, I am having to get creative...
Here is my RC4WD R3 transmission with an old 540 motor:


The problem is it is too tall in stock form to clear the hood unless I move the body higher than what would look scale/good.
So... I started a search for an internal gear to replace my standard spur gear...
I found some stuff in Australia for $200+ but that was about it until I searched for "48 planetary gear" on eBay... I found this:

Inside the rear wheel is a gyro that looks something like this:

I wondered if just maybe the planetary gear for that system would work for me... then I found it in metal on eBay for $15 shipped.
It was 48 tooth versus my 64 tooth spur but I could not tell how big it really was. Nor could I tell what the gear pitch was.
So I gambled $15 on it, hoping for a 2" outer diameter & 48 pitch to match my current spur/pinon combo.

BEHOLD!

2" outside diameter, 48 pitch, 48 tooth! Even the hole diameter was something I could work with.

Here is the slipper that needs to be grafter to the internal gear:


So I got to work. I had to remove the shoulder around the center bore & then sand it all flat & then drill out the center with a 3/8" drill bit.
The larger opening is not quite circular; it looks as if a one-way bearing is supposed to go in there but most of that material was removed so it's not a problem.
I then drilled the (3) 9/64" holes for the screws:


I did some test fitting:


You can see the one problem at the lower right of the above image...
When I had it clamped in my vise multiple times I must have missed the disc portion & clamped right onto the ring only... it looks to have cracked!
I assumed it was hardened steel like what I am used to seeing, but as I looked closer, I think it is just a cut above pot metal. A gyro would not require hardened steel.
I have to order another gear since I learned on this one. I'm nervous because this will TOTALLY work if the gear can handle the torque of the motor.

More low-profile (Before & after):


It dropped the motor about 3/8" which is all I needed. Remember tho that is actually 3-3/4" in 1:1 scale!
I just have my doubts the gear teeth will hold up...