Depending on your city, there are ~~microworkshops~~ co-ops you can go visit full of troves of different second hand parts. You can rent some time there (~ 6 €

Depending on your city, there are ~~microworkshops~~ co-ops you can go visit full of troves of different second hand parts. You can rent some time there (~ 6 € for an hour) and they will give you all the tools you need to experiment and play with your own bike