As you have an early engine, the big end diameter is normally 2.250" STD. These engines (in common with the CS 3-1) use thick wall white metal lined bronze big end shell bearings.
Lister's soon changed the crankpin diameter to 2.500" , and by reducing the wall thickness of the big end shell bearing, the connecting rod required no changes to the big end bore.
Your only realistic option will be to have your existing big end shell bearings re-metalled with new white metal, and machined to suit your reground crankpin diameter. As the chances of finding this type of CS big end bearing especially in an undersize are very limited (This will only be available as a genuine part, as the Indians never had any dealings with early 5-1 and 3-1 engines, you might like to try Sleeman and Hawken, but it will be a long shot).
Another option to consider would be to change the crankshaft for a later version (as they are interchangeable), but only if money, and originality are not a problem. Depending upon the main bearing condition of your crankshaft this might still be an option to consider. As main bearing journal wear cannot be corrected by undersize grinding, due to the design of the crankshaft.
If your engine is an early one, it will not have a chrome plated bore, and also the cylinder block will have no locating boss on the crankcase end.
Its also quite possible that you have a wet injector (so be sure to drain the cooling water if you ever need to pull the injector), you should also have a feeling pin on the injector.
The early CS engines have alot of unique features not found on the later engines.