The
single cylinder Yanmar pump is a rather simple device, a piston that gets low pressure fuel in from the
fuel pump through a one way valve, then pushes it out under high pressure through another one way valve to the injector. The injector has a spring and a ball valve that keeps the fuel from dripping into the cylinder or entering under less than the design firing pressure until the pump has provided fuel at high pressure at the top of the firing cycle. I have had mine apart several times. Pay attention to the springs in the valves and follow the diagrams in the Yanmar
repair manual and you should have no problems getting it out,
cleaning it and getting it back in.
Worse that can happen is that you still don't get high pressure pulsed fuel and still don't have a running
engine. This involves one more take off and sending the pump off to a
repair shop. Still, you might just find something amiss when you disassemble and save yourself the shop bill.
Nothing venture, nothing gained.\
Good luck