1949 G9 first start start

[Vlad]

I just bought a 1949 G9, from a friend and mechanical mentor, since he had to move out of town. About 5 years ago he disassembled and remade the engine, but never started, last year he decided to adapt a modern bike electric starter, but never finished.
I tried to kick-start, I can hear small exhaust explosion ( probably from cylinder compression ) but I'm afraid to damage the engine.
I need advice on how to proceed.

From sunny Brasil,

Vlad