I don't have a loose idler shaft handy, the one I have here is stuck in the 101 cover but the shaft stubs seems to have the same diam, depth and key in both early Scout and the 101 cover versions. An estimation would be the 101 Scout shaft bearing area length would be in the region of 32,8mm.
The early gear (40 cogs or 75,5mm diam x 27mm.) is 3,1mm narrower than the later Scout gear (36 cogs or 68,3mm diam. x 30,1mm).
The staked, hardened thrust washers are the same in both covers, 44,3 x 20,8mm x (1,4mm in this case).
The bearing distance bits that I have here are roughly the same OD 30,3 and ID 20,9, but width are 10,8 resp.7,9mm.
In the case of sideplay for the idler gear, I don't think that is crucial to the N-th degree, as long as it is not too small. If the washers shows wear, many times they are just flipped over. A tad bigger is better also for bearing lubrication. With flipped washers and without a gasket between the covers, with the used roller cages I got and the 10,8mm wide distance, I can get a 1mm feeler between a cage and distance. With the gear assembled I can get a rough measure of 0,6-0,7mm total side-to-side play.