You mean Roman Numeral 5?
We call that a model "Vee" as in the alphabet letter "V".
The only person I know who can help with a complete engine is Bill Williams, but he is in Baldwyn MS. I do not have his phone number.
Those engines are Hercules model ZXB and they were used in other equipment besides Avery tractors. I know I looked at one in a Military application power unit about 20 years ago.
Another source may possibly be Postma Brothers in Linden WA?. They were a MM dealer, I think.