[rotorwrench]

It's better if you have an original to compare it to even if it's badly rusted. Since that wouldn't be an option, I would choose the area to be sectioned to find a spot where the dimension will still be pretty much the same in shape and cross sectional size for both pieces. The area to cut should leave some similar area left on the other piece then fit it side to side to get an idea where to cut the section out on the second cut keeping the metal to metal cut joint as close and as tight as possible. Fit it up for best fit then tack weld it together with final weld up to follow. A person just needs to make certain they have all the correct related parts to start out with before fitting it all up. Fitting a part to another part that won't fit equals two parts that won't fit. Measuring twice and cutting once is for wood. On steel, cut it a bit long and grind it carefully till it fits.