Richie the original M20 taps were 1/8th BSP. By now most of the adapters in the tank have been changed or drilled and tapped to accept the more readily available 1/4 BSP taps.
Your options are :- replace the adapters and taps with 1/8 BSP or simply re-solder 1/4 BSP nipples with 1/4 nuts to your pipe......Usually available from firms like "Vintage Supplies" and " The Green Spark Plug Co" Ron
In a normal jumble time, Ron, damaged fuel taps are around. People don't bother to put them on eBay though. I only have 1/4" Norton stuff so I can't help.
I spent a good 45 minutes, rummaging and checking threads and came up with four of the lower 1/8th Enots fittings.....(Some variations). I would sell them as a lot for £20 +£3 post. Ron