Hi Amnon,
The REST API is still somewhat limited, and Get Lead by Id does indeed return a small subset of the fields from a Lead record.
You may want to explore the
SOAP API, instead of REST, which allows you to fetch all of the built-in and custom fields via a
getLead call. It is a little more complicated, but is more mature, and there are more examples around the web for complex operations.
There are also Java code examples and a simple library provided by Marketo in this
Github repository.
I hope this is more what you were looking for; if you are accessing the API programmatically, the added complexity is likely not a huge issue.
Best,
Kyle