Newbie questions about custom builds

Recently got into motorcycles and I am quite fascinated with these light/small 125cc custom builds.I got myself a Brixton BX125X to learn to ride on and eventually custom build it to my own hand and join the Distinguished Gentleman's Ride.I was first looking at a Honda CB125 or smiliar motorcycle but in these areas here it's hard to find them. Then I recently discovered some custom shops doing custom builds on cheap Mash/Bullit bikes. Basically changing everything except for the frame, brakes, transmission and engine.So I thought maybe I could change my Brixton instead of finding a Honda but the Brixton has electronic injection coming with an ECU and a lot of sensors. And I don't feel like wasting warranty on a newly bought bike.Now I have some separate questions:Is it advisable to work on a bike with an ECU? Or should I find one with a carburetor?Changing the exhaust often asks for readjusting the carburetor. Does it also apply on a ECU? If yes, how?What are popular bikes to do custom builds on which aren't too expensive (max 1500 EUR)Thanks in advance! via /r/CafeRacers https://www.reddit.com/r/CafeRacers/comments/95uivn/newbie_questions_about_custom_builds/?utm_source=ifttt