Compared to the cost of the Continental engine and the rotor the cost of airframe metal is insignficant- this includes the fact that (if your experience is anything like mine) you'll probably fabricate almost everything at least two or three times along the way. =)
You can get the 4130 tubing and sheet you need for your project from Aircraft Spruce, Wicks, and/or several other vendors .. I bought six flavors of 4130 (bushing, round tubing, sheet, and square tube) from AS last week and all were in stock. Even if you find a great "deal" on metal you still need to pay something for it and using a vendor who will let you order small lots, will take returns, and can provide you with certified sources has significant value in a scratch builder cost/benifit equation.