From your picture, it appears as though a sleeve has been fitted to where the original Bearing Journal was by turning it down and then affixing the sleeve by either press, or shrink fit, or by gluing the sleeve in place.
The prepared journal looks like quite a coarse feed rate was use. If the sleeve was pressed on you would see evidence of the crests of what is effectively a fine screw thread, smeared to some extent due to the ID of the sleeve being a smaller diameter than the OD of the turned journal. I don’t see that in an enlarged version of you picture, so there is a possibility of a Shrink, or Expansion fit, or the sleeve has been glued in position using a Bearing Retainer. However, the overall job looks fairly poor, so I doubt that whoever did the job would have taken the trouble to execute either a Shrink or Expansion fit of the sleeve.
Given that I can see no evidence of there being an interference fit with the first part of the Main-shaft journal, it could be that area has been machined down to be slight clearance fit with the ID of the sleeve so that the interference fit is only where the sleeve needs to be (makes the press fit just that little bit easier). Therefore, your biggest problem maybe in obtaining an off the shelf sleeve of the correct ID and OD for your purpose.
Correct fitting internal sleeves of various lengths can be purchased for Drawn Cup Needle Roller Bearings; for example, like the Bearings and Sleeves used in the inner bearing system of the rear suspension wishbones. The diameter the journal of your Main-shaft has been turned to will determine if you can use an off the shelf sleeve.
In the end of the Main-shaft journal you will find a threaded plug. If you remove this and you should in any case to ensure that the oil gallery is clear, you will be able to use the same Centre used when the journals of the Main-shaft were ground; use this to support the end where the new sleeve is to be fitted. The other end also has the centre used when the Main-shaft was originally ground. This can be used to carry out the machining between centres and should set the journals of the Main-shaft to run true, or you can grip that end of the Main-shaft in a 4 jaw chuck and dial the journal closest to the 4 jaw chuck to run true. When the Main-shaft is set either between Centres, or Centre at the end to be machined and the other journals running true using a 4 jaw chuck, check for any run out of the previous machining of the journal to be machined. This will determine what will be the max diameter you can machine the journal to and if an off the shelf sleeve will be available. This journal when finished must run true, otherwise all other journals carrying gears will run out, resulting in slightly different mesh characteristics of the Main-shaft gears and the Lay-shaft Cluster.