Hi, I have just gone thru 6 moths of research and thought about this. And done it.
1) Replace the steel backed bush with a Kebblewhite one piece phosphor bronze.
2) You MUST have the ovality taken off the timing side side crank bearing journal BEFORE you attempt bush sizing. You need to know how oval or worn it is under standard. There is no need to have it ground to a specific -010", -020" etc..... just whatever it takes to eliminate any ovality in the journal.
3) You then choose the correct size replacement bush.. i.e. if you have .006" ovality, you choose bush +0.010". If you have 0.0012 ovalility or undersize, you choose bush +0.020" etc and you will BORE DOWN to your custom regrind size +0.0005" you have determined.
4) You CAN use a milling table with the correct equipment (A boring head) to vertically "LINE BORE" an 107/A10 timing side bush in perfect sizing without using more than the Drive Side BEARING RACE as the datum surface. It's an unusual but clever method fully explained and shown by a typical old time British machinist on Youtube if you follow this INGENIOUS link
https://www.youtube.com/watch?v=Lg5AcTqq_9g5) Otherwise you will have to use a machine shop to LINE BORE your bush horizontally using a mandrel /pilotfixture they will have to make up to align the drive and timing side and use the drive side bearing face as a datum..you can't use the DS bearing, because its a roller bearing and will come out of the case as the line border device moves inward! If it was a roller ball grooved bearing, like Triumph T100s, this would be no problem.
6) I am very wary how hand reaming will work out well, unless you had the special sliding reamer BSA used to use ( and their dealers) which passes thru the drive side or else the reamer which used a special engine plate which affixed to the timing side ...in any case brass and bronze do not like being reamed and chattering/gouging may happen, even if you had these tools unless you were experienced in reaming..
Please contact me if you wish to have the data for the various standard sizes of BUSH and JOURNAL for the A7. It's available in chart form online and you may already have it.
I had this job done after very careful consideration and achieved the 0.0005"~0.0001 clearance and perfect crankshaft SPIN after bolting up the cases with the AG engineering method.
If this job is not done right, problems multiply and cause extensive strip downs and re-dos. Get it right for your peace of mind!