I know a lot of people like smaller lighter bikes, but personally I like to feel some meat under me when I ride.
You know, a long steel bike with fat tires ;-}
The Macargi Pantera 7-speed is a bike like that and with front and rear V brakes and fenders a pretty good bike to motorize for $230.
With just a 48cc I urged him to let me Jackshaft it but it wasn't in his budget so he went with an X chamber so at least that helped.
The front tube is farther away and fatter than most but just using 2 U blocks and an alternate clamp, and then lowering the chain guard mount in the front, the motor sits in nice and low with room to spare.
The best part, as far as direct drive goes, is the horizontal dropouts and non-fixed derailleur/tensioner.
Drive chain tensioner? We don't need no stinking drive chain tensioners!
(Blazing Saddles 'badges' reference hehehe)
Just loosen the 15mm nuts and pull the wheel back when needed, the derailleur moves with it.
Speaking of the wheel the rag sproket went on dandy with both rags and dished in, the hub is almost the exact same size as the sprocket hole.
You have to do some fender and chain guard work but nothing too dramatic, and the chain has plenty of tire clearance.
Those pictures were after it got a 3 mile run and re-adjustment and the whitewalls are still white ;-}
One tip about fenders...
Put some epoxy on the L brackets and bolt holes on the inside of the fenders, they can and will vibrate some and fail if you don't.