Here's a description and link to a plug like the one I used. I'm not sure if was a 1/8 plug. I used the smallest one I could get that would fill the hole. I have the old head in the basement (I replaced it during an overhaul due to that at the lifting bracket). Let me know if you want more details.
316 Stainless Steel Hex Socket Plug, MNPT, 1/8 in Pipe Size - Pipe Fitting
Item #4WPL2 Mfr. Model #4WPL2
UNSPSC #40142313 Catalog Page #2782
This GRAINGER APPROVED hex socket plug is 316 stainless steel construction with a higher nickel content for better corrosion resistance.
https://www.grainger.com/product/4WP...P7A1P:20501231