The studs are not the same spacing for the jug on the P-85...Plus I have no idea if you would be able to attach the 52mm piston to replace the 47mm piston that is on the 60/80cc on to the existing crank rod.
You would not get the same gains of the Phantom even if all this could be done.
This is how I would re-do a Zeda 80 to make it into a higher performance motor and I would also change your rear sprocket to a 36 tooth for better top end gain.
Now available, the all new Phantom 85 replica performance exhaust pipe made for Zeda 80 cc & 100cc motorized bicycle engine kits, features sound insulation.
www.bicycle-engines.com
Just make sure to get the gaskets necessary as well...lol.