Dunno WB. I swap engines around a lot and is pretty much just an install with a teardown thrown in.
Your Univega looks awesome. I was going to suggest a bent seat post as they really make a world of difference for comfort and gets you off leaning on your hands and the peddles. But would add height to the seat.
Spooky has them. So cheap, might be worth a try.
Spooky Tooth Cycles - Motorized Bicycles: Gas Powered and Electric Bikes - Bent Seat Post with Support 1"