Ok, it looks like a standard 2 transistor monostable multivibrator tacho, i've seen heaps of them over the years. The lower 2 diodes are signal diodes, 1n4148 is ok for these. The top diode is a zener to give the regulated PSU voltage, iit goes from +pwr to ground and you have it reversed.
If that is the diode you replaced you need to find a zener, try something about 9.6v or so.
You got the transistors wired wrong, for Q1 its base receives the input pulse from D3, so you should reverse C and B, then E is to ground which is right.
With Q2, make E go to ground, and C go to R3 (and out to the meter), so reverse C and E.
You did pretty good drawing the circuit out, it's not an easy task.
If you want to tidy it up, google for "2 transitor multivibrator monostable" which will show you a neat way to draw the 2 transistors.
These 2 transistor tachos are typical in old stuff, each input pulse from the vehicle coil just triggers the monostable, which makes a fixed timed period. Then the output is integrated (smoothed) and fed to a voltmeter, so the average DC volts always represents RPM. Hence they require a regulated power supply voltage, which is the job of the zener. You can improve its accurace by using a 7808 (+8v regulator) instead of a zener. Some of the "better" auto analysers used a 3 pin regulator to power the monostable.
If you have changed the zener diode the calibration will be off, the trimpot R11 controls the timed period of the monostable which might be enough range to recalibrate it. If not you might need a different zener voltage value. It's unlikely to be a zener less than 6.8v, or higher than 10v.
If you are using this on a different engine you may need to recalibrate it anyway.If you need a large change in recalibration you can change C1 which is the monostable timing cap.
Hope that helps.