There is this statement on the IBMWR site about R90 coils:
I think the issue comes down to three things. 1) You should use a matching set of coils. They should be 6v types with about 1.5 ohms each in the primary circuit. In series, that will be 12v and 3 ohms. 2) The total resistance from one spark plug cap to the other cap should be around 12-14K ohms (with the 1K ohm spark plug caps) or 20-22K ohms if you have the OEM 5K ohm cap. 3) There may be an issue with fitment under the tank. I've read that VW or Bosch Blue coils could be larger than the stock coils.
'78 R100/7 '69 R69S '52 R25/2
I asked for a coil for a 63 VW beetle. They were both a bit longer than the stock coils but there is enough room for them under the tank.
One brand fit in the stock bracket and the other was slightly larger in diameter so I secured it with zip-ties.
I ran these coils for two years before putting the stock units back in. (problem was the points)
I would take the stock coil with me and make sure you get the same diameter.