Maybe you can space the hub to the right to solve the clearance problems-
go to a smaller freewheel, or a single freewheel?
Also a smaller motor sprocket would give you more clearance.
I know the clearance on diamond road frames is tough- last year I was buliding up a schwinn traveller and when I found that the clearnce was going to be an issue (didn't seem insurmountable though- is your hub ground so that the motor sprock fits over it-? it's easy with a file and just spinning the wheel with pedals)
AND the tank was an issue peddling- I was using kit- tank,
I ended up finding a light Huffy cruiser and putting 700C wheels on-
heres a three cog freewheel.
Sprocket Wheel Freewheel 3 Speed for Scooter Bicycle - eBay (item 250739500504 end time Dec-14-10 21:32:02 PST)
with a single you may need a pedal side tensioner, or 1/2 link.
Of course the wheel dish would have to be compenstated as well, but if you can get the motor sprock to the right, it may clear- Is the sprocket offset? The red sprocket in the flip/flop link above came standard in a kit (it's 41t) It would give you another 1/8" at least to the right if you now have a straight sprocket not offset.
that tensioner there is a piece of work.
I'm glad your bike is back!