Welcome to Serbia, Belgrade, and this forum! You're in the right place because the crew here is awesome. You can find all sorts of things on this forum, and if you're up for a ride with us, there's a section where people organize group rides—just arrange one.
As for cafés, as far as I know, Asterix in New Belgrade has been demolished. You can check out Moto Platz at Makedonska 4 (downtown Belgrade, near Dom omladine). However, you'll find the most bikers at the free moto workshop organized by the Association for Motorcycle Safety – https://abm.rs/smart-moto-workshop/.
From what I see, the next one is on April 5th, so come by—you’re more than welcome!