Does anyone now if there´s EV-conversion kits for sale for DIY, and if so, what the prices are?
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There was a company in the UK IIRC that had designed a engine/trans/differential assembly that was close to as drop in as one could get right now. I dont remember the company name, i used to be extremely deep into EV drive systems both for my hobbies, and previous job.
Back in my high school days, i used to design and test with some pretty wild electric drive systems and built up some hot rod golf carts. For some time, i had one of the fastest electric golf carts in Canada in the mid 2000's, but as life went on, that hobby and time to do it diminished. AC drive systems wernt exactly mature enough yet, that's where the insane power/pound ratios really started to get excited. They really began to hit the "after" market in the late 2000's.
Nowadays, there's some pretty wild "semi assembled" AC kits on the market, and the influx of high amperage lithium bats, the possibilities are almost endless if you have deep enough pockets. If you want to do some homework, check out systems based on the Curtis 1238 controllers. Some companies sell kits with a corresponding motor based on the voltage and HP ratings that you might want.
Now in regards to electrifying the 500. Again i am in no ways dismissing EV tech and EV vehicles at all, but i personally have a strong dislike to seeing the 500 electrified. If you want to do a EV 500, by all means, go for it!

But with the 500 being a special car in its own right, i see it as a driving experience, a special one at that. And removing the simple gasoline engine literally removes a massive part of the soul and experience that is driving and enjoying a classic 500. The feedback, the noise, the power (lack of lol), is to me one of the main features of the 500 experience. Learning to finesse all the power from its 2 cyl's, planning shifts based on terrain, etc. Electrifying it, removes it all, and your left literally driving a tin box with some road noise.
Having experience a few "classic car" communities, the fiat 500 one really stands out as everyone does their own little mods and tweaks to these little nuggets, and there is minimal disgust towards what others do. Unlike a other community i encounter regularly (cough cough classic Mercedes purists), we all support each other even if things dont go by our own personal tastes. And i don't want to dissuade anyone from doing what they want from my opinion above.