When I had to replace my speedo I searched all I could and I remember seeing a lot of questions on using the Acewell speedo sensor with a Motogadgets unit. No one seemed to be able to get them to work together so I chose to get an Acewell and have been very happy with it.
Also, the charging light is required for the charging circuit to "activate". Without it you will just drain your battery. A resistor can be installed and there is a nice write-up by Montmil in the R65 FAQ section, although I can't just scroll back to find it anymore. If you do a search of just the FAQ section and use the words "charging " and "resistor" and search all time frame, it pops right up. I have had it installed on my R65 for a couple of years now with no problems and just use a small led for a visual indicator (the led does not have enough resistance in the circuit to allow it to energize.)