I don't know nothing about ancient languages, so I'm afraid I can't really give you suggestions. However, I feel flattered that you choose to focus on Old Frisian. I have never learned Old Frisian, just because I'm not interested in extinct languages. I am a native speaker of Modern Frisian though, so if there's anything I can do for you to help you, I will.
I don't know what you have to do, but here
, under Aldfrysk
, you can find some Old Frisian texts from before the 16th century.