I think you'll struggle with this one - there's nothing RTR that's remotely close to "Sambo".
You might be able to produce a hybrid version from various RTR bits, though. I'm not aware of any reliable drawings since the real thing was itself an ad-hoc hybrid of old parts.
The saddle tank itself looks close to the one on the very old Hornby "Caley Pug" (which was oversized), so a cheap body shell could give you a start. The footplate could be derived from the old Hornby GNR J13 tank, maybe with some repositioning of splashers.
Not sure which RTR model would provide smokebox boiler and firebox - "Sambo" is quite unique in this respect. Likewise the cab is a mish-mash.
Here's some bare dimensions:
0-4-2ST....
Sambo Number 842 (not carried) Class L2: 1914
Constructed from spare parts and used as Inchicore pilot. 4ft 6½in coupled wheels, 16 x 20in cylinders, ....
The only RTR 0-4-2T chassis are those for the GWR 14xx, which is too long for Sambo, plus the wheels are too big.