We have been using three different grits of flap discs for the last few years. They are great at returning metal back to smooth and shiny. Most of our cut-off work has been with 0.040" Metabo wheels. Some days we are armed with three 4 1/2" angle grinders, two with flap discs and one with a cut-off wheel. I buy whichever brand flap disc the local steel supplier is big-on that week.
Cool bike, nice work. I'm watching the engine work, good images. Thanks for taking the time to post them.
Hope this helps,
Steve