I don't have 'Bill Mackenzie' but I do have C vernayi (syn orientalis) L and S 13342, the original lemon (orange) peel clematis and the best of all the yellows in my opinion. I can send you seed if you'd like to try it.
However, as said above, these species are deciduous and my own seedlings at present look quite dead but I expect tiny points of green any day now as we're having a mild winter.