RE: engine prop size
Hi,The motor looks like a old k&B /valco motor. Your scale if you doing 21" that would be 1/12 scale.I have a Miss Budwiser hydro,it's 27" long by 14"wide.That 1/12 scale. Trust me and look around ebay at boats and you'll see them. The lower the number of your scale,the bigger your boat will be.All the boats that I'd seen on Ebay with.21 in it are 27-36"long.Plus I have a 30"long hydro too with .21 in it and it looks like it belongs in it.The 21" long might be a little small for your .21 motor.The motor looks very old,I hope it run for you.[8D]