Billiat and Castro are currently in the final few months of their contracts, with the duo set to seek greener pastures after negotiations reached an impasse.
Castro has been reduced to a squad player of late, having started just seven games in the last 12 months, as Mosimane begins to phase the Colombian out of his plans.
Meanwhile, Billiat came off the bench for the final 30-odd minutes on Tuesday evening and bagged both goals in Downs' 2-0 win over struggling Absa Premiership champions Bidvest Wits.
Mosimane has since been building ties in South America after going in search of replacements for the pair during the recent international break, as he starts preparing for life completely void of the once-popular 'CBD' trio.
"We're still there [in South America], I've got people there, hopefully we can close a deal, but it's not easy," revealed Mosimane.
"Khama is leaving, Leo is leaving. The CBD is gone hey, so we have to start something [new]. CBD is gone – Keagan [Dolly] is gone, Leo is going, Khama is going, so we need to change the team."
Midfielder Tiyani Mabunda is also in the final months of his contract, but 'Jingles' expects the 29-year-old Bafana Bafana man to stay put as negotiations are ongoing.
"'Shuga' [Mabunda] will stay – he wants to talk and discuss terms, so he'll stay. Where's he going to go? Which team does he want to play for? I mean he can choose to go to another team but I don't think [he will]," said Mosimane.
"I think he's clever enough to know that he has to stay here and play for us. He's got Champions League next year, and we give him a position in Bafana, so does he want to leave this team? I don't think so."
Mosimane further explained they are going through somewhat of a rebuilding phase at Chloorkop as they prepare to have another right go on the continent next year, but insists they are still capable of reclaiming the domestic league title.
"Someone recently wrote a very good article that we should let the coaches rebuild their teams... What a briliant article, but is it easy to rebuild? The supporters want the win, there's no excuses of 'we want to rebuild the team'," he added.
"But we are doing it slowly, and I'll bring one or two, preparing for the CAF Champions League which starts in three months' time. So it's not only about [the] local league.
"I think we have enough material for the league, locally, no excuses. Not to say we are blowing our own trumpet, but this team is good enough to really play and win the league."
The Brazilians have since moved up to fourth position on the log, and now trail leaders Baroka by just three points, while still having three games in hand.