There is no "syntax", it's all just key=value pairs, and all the subsystems have their own set of keys/directives, and the values have their own mini-DSLs. Things that end in "Sec" (for "seconds") take duration labels. The only directives that are shared is the inter-unit dependency directives. Some keys/directives can be specified multiple times.

I don't know why you'd be parsing unit files or serializing something else to unit files. Just drop them into place. The hard part is knowing all the details of how the directives interact and what their values can be.