Those tanks were made by Chilton, They were a universal tank. Few years back someone blew out a pallet of them for like 15 bucks a piece out of a closed Hardware store auction, Heres the thread I posted when I found them:
https://oldminibikes.com/forum/index.php?threads/steal-of-a-deal-round-tanks-14-99.105389/ they are later tanks than most of the mini bikes, Minibike world used them on the Flee's but they were built in 2000's and only produced like 200 bikes. Other than that they were more like a replacement tank for equipment for both fuel, hydro, and fogger systems
Outlet is standard 1/8" npt