If the engine comes with a tank, use that one.
Positioning a fuel tank for engines mounted high can be an issue.
This is especially if you're running external tanks or auxiliary tanks. Gravity works against you.
I've bolted plastic goped saddle tanks aside the friction drive housing. I've also bolted plastic tanks atop the housing. They worked ok.
At one point, I installed a 2-liter peanut tank on the slanting tube of a girlie bike, to feed the rear friction-driven engine.....and the front friction-driven engine.