I have 3 Hornby Class 121s which I am currently converting to EM along with a number of other diesels. I am using Ultrascale drop in wheelsets which are on order. Lead times with Ultrascale are long but product excellent and worth the wait. Around £25 a set.
I’ll upload a ‘how to’ if it turn out to be much more complicated than drop in. I will also be adding internal lighting and direction and motion sensitive cab lights.
Hope this helps.