Seems like we have a winner: valves. After adjusting valves, still didn't start easily. With some throttle managed to get it running and after 20-30sec, eased of the throttle and it kept on running. Battery is quite weak so it still starts hard, will have to go for a spin later to charge the battery. The reason why valves have started to get loose is wear on rocker arms, probably makes sense to change them soon.