Not sure if that would work, the 90 would appear to be a round dial that drops into a faceplate. The IIa has it built in to the Left dial along with the fuel gauge and main beam lamp

Thanks for the response, being an exRAF 109, the ammeter is built into the left dial so not an extra! Will have to think about rewiring or removing!

Somewhat new to this as just acquired a Series IIa. Fitted with basic 16ACR (why upgraded to this from the original dynamo rather than something beefier at the time ??) so was considering the upgrade to something more substantial. As we live in the country in Northern Scotland, infrequent use...