SMLE tolerances are not as bad as some might think.
According to my information. the total tolerance on spacing distance between the holes in each pairs of rivet holes is quite small. When I dig out the file I will post the dimensions and tolerances for tha receiver and bridge holes.
Note that there are two different rivets involved.
The trickiest part would be setting up a suitable jig that would allow the rivets to be peened into place and prevent damage to the receiver in the process.
The fact that very large numbers of L E rifles were upgraded to SMLE configuration, including the attachment of a bridge. Not for the faint-hearted, but not impossible.
There should be a few charger bridges floating around from all the SMLEs that have been cut up by gunsmiths and dealers in Australiain the last fifteen years or so.
Rivets can be easily turned up on a basic lathe. Soft-solder would certainly help if you are planning to get a bit rough with the old girl.