If you can not get an MC11 piston , fine one that is neares, note positions of ring pegs. ( they need to be the same or very similar. Do not worry too much about there being no piston skirt window this can be cut very easily. Make a template and reference it from the old piston crown. Then carfully cut new window in piston with die drinder ( dremell) make sure the window is very smooth and radiused in the corners and polish off any "machining marks".
I have done this many times when racing a 500cc maco in MX side car racing, I NEVER had a skirt failure as a result and my 500 reved to 16,000 rpm.
Try looking in Mahle piston catalouge or Karle Smitt pistons.
Good luck,
Dave.