I am gussing that the vavle or lifter inside engine was broken.
You must drive a very old car, instead of fixing, it's better to change
the engine and repair. Uesd engine for old cars are quite cheap,
if lucky, you can find much lower mileaged engine from accidental cars.
Old engine could cost as low as $150. Labor costs about $500~700.
Maybe $1200 as that garage told you. Install an engine is simply by
swapping the "new" into the engine bay. Fixing an engine, the garage
may hire an expert and spend 10 hours to take apart the engine and
repair and put everything together.
one cylinder as that garage told him. (他说是我这个缸里的一个什么东西断了,修的话要1200元). Injector is a metal, non-moving part, how to get broken?
anyway, that's just my guess. Guess is a guess.