It sounds like a bare alternator, rather than a fully regulated genset.  If so the output will be quite droopy and revs dependent - which may suit the application just fine - such things are often used on farms for pumps and random machines in funny places and 220v+/-20%  at something in the range 40-60Hz is not that serious.

I'd add an RCD, verify there is an NE link, add one if there is not and then fire it up into some heaters or something and see what it does.

Do you have model no and maker - we may be able to find out a bit more about it.

Mike.

PTO generators operate with an input shaft at 540 RPM, the standard PTO speed. This should give 50Hz out, and the quality of regulation will depend on the exact design, but may well be quite good. The tractor Tacho will give an accurate reading of PTO speed, as it is used for all the attached machines, and the engine governor should keep the speed within a few RPM. The Earth rod should be driven in the ground as usual, and I would expect a neutral connected to chassis and available on the output socket. If not come back for more information. If you do not have a tractor of reasonable size you will have more problems, as the alternator has gears fitted to give an actual shaft speed of 1500 RPM, and expects a significant size engine, these machines are often 40kVA or more. More details needed and a photo would be good.