Re; “If it’s magneto that won’t be possible but you could use something like a Scitsu.”
Tut..Tut – it’s not a timing cover you need, as the mag is already reduced to 4:1-what u need is a 1:1 rev counter drive, like on the BSA in pictures, fit a big nut with suitable slot on end of magneto, fit clay/plasticine into top of timing cover push on and then drill in the centre a pilot hole followed by a bigger hole and drill the two holes for the bolts, welding two nuts on the inside.
The 500 Gold Star shows how its done;
http://onlymotorbikes.com/public/16/bsa-500-ss-gold-star-1972-moto.jpeg
https://www.bing.com/images/search?view=detailV2&id=590026BA31E2BEA89C2B01F519F10D22CA82866F&thid=OIP.U7jLk-Gwi1x1fuiKkLckJwHaFj&mediaurl=https://eglivincent.files.wordpress.com/2013/12/10.jpg&exph=857&expw=1143&q=BSA+500+gold++star&selectedindex=16&ajaxhist=0&vt=0&eim=1,2,6
you need to establish the centre of the shaft in relation to the timing cover
assemble the cover with a blob of modeling clay ,then remove and measure thickness , remove nut on mag drive and get someone to turn you up a
pointed nut a couple of mm longer than
than original nut + thickness of modeling clay fit piked nut assemble case and ding the aproximate area with a mallet this will transfer the centre of shaft via the nut /centre punch !, drill a hole a small pilot one for now as a reference to aid marking out
now you may have to build up a bit of a platform by marking out the base of your drive unit and build up the area by tig welding ,or i suppose you could use jb epoxy if youre a cheapskate.
now drill a clearance hole for the spindle and spigot of drive ,spot drill thru the unit f
or mounting holes ,and drill thru and tap
get a forked drive nut ,3/8 bsf for mag end goldstar? or if using bottom sprocket 3/8 bscy inter /manx ?
or make one
check that the spade end is not bottoming in the fork when fitted again plasticene /blutack will establish this
well thats tin shed engineering for ya