Xyzer (Dave) was the one who sorted out the whole issue of failing idlers and the error in Rajkot idler bolt hole placement compared to Lister CS many years ago. Xyzer made a few batches of large offset bolts - my 6/1 needed something like 0.055 inch offset. They were very high quality work, as were the hollow dippers he made, which I also use. It was a hugely generous contribution he made to CS clone/ Listeroid users; something like 14 years ago. His contribution solved the problem and failed idler gears were no more.
The offset bushing seems a good approach for the diy solution, as it lets you test fit different offsets. Boring out the case hole is the only way to do it for larger offsets. I'd bore it to a standard size then very carefully drill offset holes in stock round pieces via carefully setup drill press, or use my wood lathe and 4 jawed chuck for the offset drilling. Less than ideal but allowing for a lot of duds (benefit of bushing approach) it should work.
For smaller offsets you can turn down the stock bolt and add an offset sleeve. 32coupe made some very nice sleeves with turned down bolts that use the stock crankcase hole. They are a good solution for smaller offsets if he has some left. I used on for my neighbor's DES 8/1 propane conversion project and it saved the day. Much appreciated, 32coupe!
Very impressed that you did your homework, DaveS. If you have a large amount of clatter that isn't from the tappets and valve lash (finger test) then you likely have the need for a large offset like 0.040 or more. You should be able to find old posts on doing lash measurement. After doing it once, you can do it just by feel. 6 mils (typing paper) is a nice fit.