The photos below show a 741 chassis with a Triumph 4-speed. Note that the Triumph gearbox is from a pre-unit swingarm bike (approximately 1954-62). The rigid frame Triumphs used different upper mounting points for the gearbox and are a little harder (though by no means impossible) to adapt for Scout use.
Many other British bikes of the 1950s used very similar gearboxes which could equally easily be adapted to Indian use. Some of them were: BSA swingarm pre-unit singles and twins from, roughly, 1955-62. Norton singles and twins from the 1950s up to and including the Commando models of the 1960s and 70s. Various AJS and Matchless singles and twins (I think both rigid and swingarm frame models have the same Burman gearbox). Ariel singles, twins and fours have slightly different Burmans with the upper mounting hole elongated for adjustment [see photo 6 below]. Someone mentioned Royal Enfield, but they don't really match up to the Scout as well as any of the above. For more info, you can buy a booklet on the subject from Victory Library in New York.
Most of these gearboxes are probably easiest mounted rigidly in the Scout rear engine plates (like the Scout trans) and the chain/belt tensioned by a slipper/roller (like Indian used) or similar, but you could rig up something like the Brit system where the box pivots around one of the mounting holes and has a tensioning device at the other [see photo 7 below].
Using the clutch that goes with the gearbox is probably much easier than trying to adapt the Indian clutch, and you might be able to use the primary cover that goes with the gearbox too (probably needing some sort of adapter at the front end where it meets the L/H crankcase half).
The Indian and Brit chains don't match, so unless you want to make up a belt drive, you'll need to grind the teeth off the Indian engine sprocket and weld on a Brit size sprocket blank (or the other way around with the Brit clutch drum, but a #35 three row blank in that size is more expensive and maybe not as easy to find). A single row chain should be enough for even a healthy Scout engine (but go with what the clutch drum has). Lots of 500/650 Brit OHV bikes ran single row primary chains, and the major difference on the Scout is the lack of engine shock absorber (compensating sprocket) which will probably shorten chain life - but it's not hard to change a chain once in a while so I don't think this is a real problem. Besides, if you end up with a Triumph trans/clutch, the clutch centres on the alternator models (from 1950 or so) had a rubber-based shock absorber built in.
If you end up running a semi-open chainguard instead of a closed primary cover, you may want to consider adding a ScottOiler for chain lubrication. The clutch bearing rollers also need lubrication, but will survive ok with regular greasing if you rig up a simple sealing system (bits of felt and scraps of metal goes a long way if you don't have the tools to make something more elaborate). If you go with a belt drive - or actually no matter what - you will need a real oil seal where the drive shaft comes out of the engine. Both Rick Abbott and Indian Parts Europe have bearing nuts with oil seals.
If you can get a rear wheel
from the same bike that donated the gearbox, clutch and maybe primary cover,
it may be a good idea to get that as well. It will save you turning over
the Indian rear wheel and possibly messing around with chain-to-centerline
distance for the rear sprocket. On the other hand you will have to make
up spacers and axles so there doesn't appear to be any clear advantage
with either solution. It shouldn't be too hard to adapt the Indian wheel
if you choose that; mainly a question of flipping it over and designing
a new linkage and anchor for the brake. In most cases, no matter which
wheel you use, you will need to come up with a new brake linkage.
