The fuel injector is made by Hitachi (part number FBYCG80). It is also used in a Subaru automotive engine. A single injector is available via AliExpress.com (China) for around $20 USD, delivered.
The injector is a solenoid valve operated by 12-volt power. EFI engines commonly have a correction for battery voltage. The higher the voltage, the quicker the injector will open and therefore the more fuel it will inject for a given pulse width.
On the OSSA, there is an additional part that “tops” the injector to convert the fuel-rail inlet into a hose-barb inlet. The engine cutaway view shows that the fuel injector sprays directly onto the connecting rod's big-end bearing area.
The fuel injector's mating connector is a Sumitomo MT090-2 (with bottom rib).
Hitachi FBYCG80 injector with hose barb fitting
Injector firing: Trace 1 voltage, Trace 2 current (1mv = 1mA)
Cutaway view showing internal engine parts and their arrangement.