the best thing would be to get an affadavit from either one parent and one from parental uncle (usually kaka would be good as you would have the same last name). just needs to be notarized by a lawyer in nepal.

in addition to that you would need a non-avability certificate, saying that your birth record was not found when they checked the records.

i think this way is less hassale than getting birth certificate as USCIS these days send RFE for recently issued birth certificate(seen most cases but cant be 100% sure as they question how can you get a birth registered when you are adult.)

i did these things for my 485 last year.

good luck with your process.