Mechanic won't give me my vehicle back

I hired I mobile mechanic to do some work on my vehicle. I've paid them $1200 so far . Still owe $3,049 which I am an agreement with. Job ; 2020 Kia optima- Remove and replace cylinder head - replace gaskets and timing chain. They have had my vehicle since 11/20/2024. I was told via text 12/23 my vehicle needed 2 more days for pressure testing 12/30 followed up and got the same excuse. 1/4/2025 was told AGAIN my vehicle would be ready then heard nothing.1/16/2025 was told on fb my car would be ready . 1/21/2025 after asking about my vehicle AGAIN they stated NOW they were putting a cam sensor and oil control valve sensor on today ?!?! What is going on ?? We didn't discuss anything about these sensors . My question; is this something that goes along with replacing gaskets and timing chain ? I've told them I'm only paying the agreed price . They have my car boxed in on a "lift" at someone else's shop