Based on the gemara 38a, the Shulchan Aruch (128:14) paskens that one can only do birkas kohanim in loshon hakodesh, standing, raising his hands, and with a medium level voice. The M.B. also says that it must be panim k'neged panim and that most poskim say that all these things are l'ikuva and m'akeiv even b'dieved. Furthermore, the Mishna Berura writes that by all these details, if the kohen is unable to do these things even though he is an o'nes i.e. too weak to stand, unable to lift his hands, can't raise his voice, he cannot to birkas kohanim.
Rashi seems to contradict this approach. The gemara says 38b that those who are out in the fields b'ones i.e. busy with work (rashi) and cannot come to the shul for birkas kohanim are included in the bracha. Rashi (d.h. d'lo) explains that those who are present in the shul and make it obvious that the bracha is not important enough for them to come in front and face the kohanim to be "panim k'neged panim" are not included in the bracha because we require "panim k'neged panim" [Biur Halacha (128:24) writes that included in this will be people who are in their homes and opt not to come to shul are also showing that the bracha is not important to them and therefore are excluded from it. He is medayek this from rashi who describes those who aren't o'nes to be "those in shul behind the kohanim" implying that if they aren't in shul even if they are in front of the kohanim they are also not included]. Clearly, rashi indicates that the din of panim k'neged panim which is d'oraysa like all the other things mentioned, only applies when the people are making a concious decision to violate the halacha, but in a situation of o'nes such as the am sh'bsados, the bracha will be effective even though we are not fulfilling the principle of panim k'neged panim. Similarly, rashi should say that by all the other details as well, if the kohein is an o'nes and cannot do them, we will allow him to duchan anyway?
A possible distinction would be that even in a case of o'nes the kohein is not allowed to violate the any of the halachos and therefore does not duchan. But as far as the chalos of the bracha, it is chal on those who are absent due to o'nes. However, this doesn't seem to be sufficient because the gemara says that if the entire shul were kohanim they would all go up to duchan (if there isn't a minyan that remains) and the bracha is chal on the am sh'bsados. How can they lichatchila go up to duchan, it is a violation of "panim k'neged panim"? We obviously see that in a case of o'nes we allow the kohein to duchan l'chachila even though he will not fulfill panim k'neged panim. So too we should allow a kohein who cannot duchan in loshon hakodesh, or cannot lift his hands, or cannot stand or cannot talk in a loud voice to duchan since it is a case of o'nes?!