1975 Mercedes Benz 450sl 4 of the 8 injectors are working

The 1975 450SL used the Bosch D Jetronic fuel injection system. It's a primitive (cal it "early", if that makes you feel better) electronic system. You don't provide much information in your post, but here are a couple of guesses.

The catalytic converter was in the engine compartment, which cook and crisped the wires. Pulling the engine down enough to change the timing chain could have disturbed the cooked insulation, leading to short-circuits and no operation.

The DJet system doesn't do sequential (one squirt per cylinder, only when it needs it) injection, it fired one bank at a time. If all the failed injectors are on one side, you could have a failed or failing ECU. That's mounted under the dash, on the passenger side.

I have a 1977 Mercedes Benz 450 SL, which are the marks to install the timing chain on crankshaft gear and the gear up?