Welcome to the boards. The bike looks really good so I think an aftermarket tank is not the way you want to go.With a bike in what appears to be such good original condition you probably want to keep it that way. I would take the tank to a reputable radiator shop and see what they think about sealing it. You said there are quite a few tiny holes which could mean a large area of the bottom has very little metal left. The shop can guide you. I had a issue with holes and they used silver solder to patch it up...but then you will need to paint it as the process will damage the paint. There are a number of these tanks on eBay, but you have to be careful you do not end up with another junk tank. Before you go any further I would take the tank in and get an opinion from the radiator shop. Heck, you got a nice deal for $300 and the opinion of the radiator shop is free.