Thanks for the replies everyone.
So this unit I have is not a Chinese knockoff, its an Italian made machine (better/worse haha I dont know), it has a genuine Honda GX120 engine in it, and I cant find any branding on the generator alternator side of things.
Its quiet, you can have a conversation while standing next to it. I have not yet found a date of manufacture yet, and the Italian company who seems to make it 'Airmax Generator Division, Italy' does not seem to feature on Google, or has closed down. There is a name plate on the side, which is 9402 which could indicate 1994 or 2002 potentially.
Under the cover of the alternator it has an 8uF Run Cap for the field I guess, but nothing else on the output stage. The DC side has a single phase bridge rectifier, but no capacitor on its output, so that is nasty looking on the scope too. I guess that is designed to be plugged in to a battery to charge it, or into a device which has decent input filtering. When you put a cap on it unloaded, it stabilises to a more DC looking signal, but does vary somewhat when the RPM deviates a little on the engine. Its actually around 24VDC when unloaded with a cap on it, but comes down with load, much like a transformer I guess. I regulated it down to 13.8V and the output is nice and stable, just when I was testing things.
So going back to the AC side, yeah its not very nice, and I can appreciate what has been said. They are what they are, and they do power things I know. I am not wanting to upgrade for any particular reason than to have a project and improve what I have. Yes buying new will get me better I know, but thats not what this is about.
I have powered a fan, a drill, some lights etc, and no issues from what I can tell. The output on the scope however is really not great. Here is a bit of a pic of it, in differential mode with math CH1-CH2. I only have 1x/10x probes, so excuse the poor shot.

Here is one of the DC, unloaded, without cap.

The thing I don't like is the varying frequency. As load comes on and off, the motor RPM changes, and then it reacts and it goes fairly stable again. There is no comms on this unit, no feedback between the generator side and the engine, its just a governer. If you get a bit of crap in the fuel, or it starts running out of fuel or something, the revs change, and the motor might conk out if the fuel runs dry etc, but during that period the frequency is all over the place. My fear is what might happen to 'something' connected to it which is more sensitive.
I don't have a particular load in mind, its just something I wanted to bring up and see what people think.
When I got the generator the engine had bad blow-by, the carb was totally gummed up, it was leaking oil, and a few other typical maintenances related issues. The insulation under the covers powdered when you touched it. So I honed the cylinder, got a new piston and rings and gasket set, ultrasonic cleaned the carb and rebuilt that, and now it runs brilliantly. Replaced the insulation, and basically gave it a total work over, it was good fun. So I did that last week, and while I had it out I thought I would attempt to get the output to be 'better', and that's when I stumbled on those links from Aliexpress.
What I can't make out from the Aliexpress stuff is if they are Modified Sine Wave output, or Pure Sine Wave Output, and if you need any other 'bits' other than the board itself. The links suggest both modified and pure, and then some have some instructions on some sort of modification, but I cant follow what its talking about. There are some cheaper ones, and some more expensive ones. I have just found another one which seems another step up but I have not figured out what the actual difference is yet.
https://www.aliexpress.com/item/High-Quality-3000W-Pure-Sine-Wave-Inverter-Power-Board-Post-Sine-Wave-Amplifier-Board-Assembled/32888215704.htmlWould love to know thoughts.
Given the generator is 1400W, I imagine if I rectified to DC and inverted back to AC again I am going to lose some capacity. I guess I could have a switch to change between raw output or inverted output, depending what I power off it.
This is just a bit of fun and to learn something new, I have never done anything like this and I know little about inverters.
Here is a bit of a video of it running just after I put it back together a few days ago.
https://streamable.com/t77d3Thanks