The 1973 up-to mid 80's Yammy XS650 twin make a great donor bike. They're reasonably cheap and there is plenty available with good parts availability.
Yamaha XS650 1″ tube Bobber or Bar hopper frames start at £850. You can add gooseneck style for £75 which includes stainless engine mount bolt kit. They have that cool parallel twin look giving them a cult following. We’ve made quite a few chassis for these so if you want a frame or a full roller then look no further. Check out the links below for more info.