I use 6205 roller bearings ( in another thread some month ago  i mentioned a 6202, but that´s  just the Alzheimer) with no shield for at least 65000KM without any problems.
And: the oil that is pumped to the head has an exit, it´s a kind of hole/ tunnel, not so small,  leading to the camchain and down to the sump You can watch it when you open the Camcover while the engine is iddling. With higher revs you may get some dirt on your face.
I wouldn´t mind using an open bearing, there is enough oil staying inside the head"chamber".
A needle bearing has slightly more friction than a roller type, you can try it, but you won´t find one that fits  without further action, making a sleeve and so on. The 6205 seemed to be fine, nobody who did this conversion had problems, as far as i know (?),
Regards Ali