Hi Andrew,
I have a P21 and use an e-propulsion spirit 1 plus. It replaced a Tohatsu 5hp 4-stroke. The 1kW electric motor is nowhere near as powerful as the Tohatsu but I have managed well with it. The reduction in noise is fantastic. You don't say what your sailing conditions are but perhaps you're on a lake or estuary? I'm sailing at sea (Moray Firth) with only a short distance to get out of the harbour.
There are a few models available that could suit. Considerations would be the size/shape vs space available, size of the underwater part for getting through the well, the amount of power and range you need, and cost.
Range anxiety is my main worry. The Spirit1 will run on full power for just over an hour, and I get 4 knots in calm water and light wind. Speed drops with rough water or strong head wind, but I've been fine getting in and out of the harbour which is all I use it for most of the time. I've motored home when the wind died a couple of times but the conditions are good in that case. Easing off the power obviously reduces speed but the overall range increases, which is helpful as long as you're not in a hurry. You can buy additional batteries but they are expensive.
Removing the motor and putting it in a locker is easy as the battery comes off. One thing I hadn't anticipated is that if the motor stays in place then it whines when speed goes above about 2.5 knots and increases in pitch as speed increases - there is no possibility for the propeller to freewheel quietly.