Quickly skimming the motor factors website I see that 80 frame motors are typically spanning a power range of 370 to 1100 Watts. Most of them are 550W, and just a few are as powerful as 1.1kW.
When you change out a motor it's much easier to keep the same frame size as this means mounting holes, spindle diameter and spindle position relative to the mounts should remain unchanged and makes for a much easier swap.
If you were to change the motor, then stepping up to 2hp / 1.5kW wouldn't be a bad thing. Those will typically run off a UK 13A plug. But that isn't a huge increase in power and you will need to change the motor mount and bore out or replace the pulley to fit a bigger spindle, which might be more work than it is worth.
I would try the easy thing first. Change the capacitor. It has done well to last 40 years and surely doesn't owe you anything. If it is a single capacitor, it will be permanently connected and if it has failed, then the motor will be essentially running on one winding instead of two. That will reduce power so consistent with the symptoms.
Capacitors are commodity items and only cost a few pounds. Under £10. Just look on ebay. You should match the capacitance (at least to within 10% of whatever the existing one is labelled), and be sure the voltage rating of the new cap is the same or higher. If there's one in the 300V range for similar money, choose that, just because a little extra headroom never hurt.
20 microfarads sounds about right for a run cap on a 1.1kW. This is often abbreviated to 20uF because most of us don't know how to get that greek symbol for "micro" up on screen.
mF made me instinctively read millifarads which are 1000 times bigger and an almighty big can !
Your capacitor isn't 20 milli farads
Good luck.
It pays to be neat with your wiring. It is also a good thing to use properly sized ring crimp terminals on the wires (including the capacitor connections) in a motor terminal box. Please do use cable glands or at least grommets to protect your cables from sharp edges and keep the dust out of your motor, and don't use electrical tape for anything at all. If you need to insulate, buy a small packet of heat shrink sleeving.
