The P218G is the 18 hp version that came in the Case 448 and some John Deere garden tractors. So definitely not original.
Here is a used one on eBay: Case Ingersol 446 Onan BF-MS/2891E OEM 12V Electric Starter C29733 | eBay
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And some more if it is the right one: onan p218 starter | eBay
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Thank you for everyone who provoked me to act in order to solve my starter issue. After looking at the model and serial number of my Case 446, I have purchased a starter from Ebay. I was looking for a starter to fit the P218G engine . It will be delivered Thursday 2/22.
Sincerely RJC