I replace as many of the metric bolts and screws with SAE that I can. It makes things easier later on as replacements are easy to find... It also make finding the correct wrench or ball driver easier... I also replace the 2mm push rods with 2-56 or better, depending upon the size of the ARF. I will also use real axles over bolts whenever possible. I will use the supplies control horns, especially the ones from World Models, but usually replace the clevises with metal DuBro or Great Planes.