The 10 year old iCN510 is just a rebranded Mio 136 (Pocket Pc) running windows ce (4.20, I think). Navman installed their GPS software and locked the system.
It can be unlocked, I am almost sure, but I never did it myself. In the past, there used to be a website about unlocking all Navman devices, however it has folded.
Not sure about this, but their "MOOV unlock" could be the one they used to unlock the 510. If you can find it.
What should be possible to find on the Internet is miopocket. (google)
Miopocket is a heavy program that weights on the devices memory, so it may not be the best choice after all.
Then there is "danibor lanik" who might have an unlock that works.
The device is almost ideal to play with, but don't expect much.