If you know your diffuser operating parameters, you could just add a correctly sized compressor and the solar components to run that compressor.
I know that Vertex diffusers will work on .5 CFM as a minimum. Now with a DIY system, you have to do your research on the compressor to see if it will furnish that CFM to every Vertex diffuser that you have in the pond, at the depth that they are placed.
That's the differences between a DIY system and a purchased turn-key system. Turn key, all the calcs are done for you and the system should be sized correctly to do the job it is supposed to do. DIY means you are the engineer to find out what is required. At the very least you have to find out the operating parameters of your diffusers to know if the compressor will work.
For instance, if you have 4 vertex diffusers placed in 10' of water, the compressor needs to furnish a minimum of 2.5 CFM at 6 psi for it to work. Bare minimum is 2.0 CFM at 5 psi, but that's leaving absolutely no room for a drop in pressure or volume as the unit ages and wears. The 2.5 cfm and 6 psi has some fudge factor figured into the calcs.
You can run a 120v compressor on solar, you just have to correctly size the panels and batteries to do it.
