Ok, sorry, got ahead of myself. The 120 has the Deutz engine where the 150 & up have the big bore Volvo engines. Stupid mistake. The deutz still use the compression seal on the bottom of the injector against the cup. No conical seat. I believe it's still copper cups. The tooling is different but the procedure is the same. I don't know if you can borrow or rent the tooling from the dealer. Personally, I can't justify single purpose tooling for one job.
I have attached a pdf with the procedure and specific tooling with part numbers. I have also included the cyl head parts page showing cups and O-rings. Again, if you're not comfortable doing this, it might be worth farming out. Make sure you keep injectors clean, bagged and tagged with position as each has a unique trim code written to the E-ECU. Good luck