If someone used to not pray and fast, then he repents sincerely, and starts doing his obligations and avoids sinning as much as possible, but unfortunately this person dies before making up the Qadha of all his missed prayers and fasts and other aamals.

Can he enter the good barzakh? Or will he be put in the bad Barzakh until someone prays and fasts his Qadha Amaals on his behalf?

Yes as long as he sincerely repented and tried to fulfill missed obligations, Allah may forgive him fully and he can have a good life in Barzakh. But if others pray on his behalf, he would definitely be in a better place in Barzakh.