You have 3 choices really-
1. Fix yours,
You CAN fix yours if you know what you are doing (rear bearing is tricky), but for most of us it's easier to take to an autoleccie, or buy another.
Problem could be either just worn brushes, or the internal voltage regulator is dead.
2. Buy another Lucas (s/h, recond, or new)-
I bought a new Lucas for my wife's Mini, from Mini Car Clinic a while back, for $195. I figure it will be trouble free for 10 years or more.
3. Buy something else and make it work.
A Bosch 45A will pretty much bolt straight on, but the mount needs trimming with a hacksaw to give enough belt adjustment.
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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.