I know little about e-bikes, but if I was going that route I’d look at Woosh and Swytch. Both offer ‘conversion kits’, that could be used on your existing bike - and if you’re not needing the electrics, you can swap back to its original form.
No experience of Swytch but with conversion kits in general, I would consider how you want the controller to work and read some independent reviews, download manuals etc.
I converted a bike using the Woosh rear hub XF08 motor kit, which came with the Lishui controller. The kit was good quality and Woosh's aftersales support was exemplary. They sent out replacement/alternative parts FoC when it transpired I'd been sent the wrong ones and were very responsive on email. The motor was also very smooth and torquey while being UK legal.
However I did not like the way the controller worked. My other ebikes work so that the assist level varies the power delivery, but the levels all cut off at the UK motor limit 25 km/h. However on the Woosh kit only the level 5 cut off at that speed. The others cut off at speed steps under that, with around 3-4mph between them. This made it frustrating to ride on flat or gentle rise as your speed would settle to the level of whatever assist mode you were in.
The controller went back to Woosh for tests and they tried to get a firmware update but there wasn't one available and were forced to conclude it was designed to work that way, who knows why. I sold the kit on and put the bike back to standard. I later bought a Vado SL for commuting which was the main purpose of the kit bike.
I can only assume that most buyers get the throttle option and just leave it in the top mode, hence why Woosh were surprised it worked that way.
Based on a dataset of 1 I concluded that kits are a very cost effective option but there were too many compromises for me and I preferred paying more for better refinement.