From marinedepot.com's bulkhead page. Looks like that falls between 1 and 1.5"
+1 for 1.25"
I bought an HDPE bulkhead from an agricultural supply store locally. I used a 60mm hole saw for my 1.25" bulkhead if I recall. I'd have to find it and measure it to be sure.
Any retailer online should have the sizes listed. Try measuring the hole with a digital caliper.
If it is really an odd shape, it wouldn't be impossible to enlarge the opening for a 1.5" bulkhead with a 2-3/8" saw. That is if it isnt tempered
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