I haul my Bike Friday pakiT up many levels of stairs. For that reason, I purchased the Onemotor electric assist kit, which weighs 4.5lbs and comes off in 30 seconds. When I get to my destination, I pop off the motor and battery and stick them in my sling bag. Then I'm not hefting any added weight up the stairs with my arms. Depending on where you live you might see if there are some systems that detach easily (friction drives) and don't add the weigh of a hub motor to the bike. European brands include Add-e, Go-E, and Rubbee. Now my BF is only 19 lbs and I know Bromptons weigh more. The obvious ways to drop weight are super light saddle and pedals. Everything else with a Brommie is gonna cost you $$$. Maybe invest in some hand weights for the house and keep them by the tv watching chair and just do bicep curls while you watch tv. Drop a pound with the saddle and pedals and bulk up the biceps and maybe that will be good enough.