While waiting to go ahead with the engine, I switch to the gearbox: first cleaning with gasoline, first pass with a gross metallic brush, second pass with a softer brush and then finishing with a towel full of degreaser. I found a degreasing product with an orange scent, much more pleasant than the previous product I used, which was spreading an awful scent all over the house for a few days! I spent hours cleaning this gearbox. I even tried to use acid for a few parts but finally I think that brushing does a better job, at least for aluminum.
I already have hundreds of zinc plated parts ready for reassembling. I started with the gearbox, the result is beautiful. For bolts and nuts, depending on the result after zinc plating, I keep the original parts or I replace them. I bought more than 4000 bolts, nuts and washers 😉