Go for the integrated tank!
Dude!
I have laid awake nights thinking this exact thing with this exact same bike! The MP is sweet! The local Harley dealer here has one in the showroom already motorized, with a price tag of only $2500.00(no s**t).
Not all Harley riders are like this, but you know some yuppie sucker is going to pay it when he could get the $500.00 bike and $200.00 motor himself.
Anyway, I have sealed an old motorcycle gas tank with fiberglass resin, and it worked great- just poured it in and sloshed it around while it hardened. That's the technique I'd use on the tank/frame of a felt bike.
Stand the bike on its nose and let the resin harden at the front of the tank, then on the tail to seal the rear. Wait until the resin is thick before you pour it in so it doesn't leak into where you don't want it to go.
The bike at the Harley dealer uses an antique cylinder-shaped oil tank behind the sink for a gas tank, and it looks sweet, by the way.
That tank has PLENTY of capacity!
Do it! Keep us posted.
Sherm