The phrase-final, conjunctive particle 「～もが」attaches to the ren’youkei of nouns, substantives, adjectives, the copula and auxiliary verbs. It has several related and semantically similar derivatives that have added emphatic particles: ～もがな, ～もがも, ～もがもな, ～もがもや, and ～もがもよ. 「～もが」and these variants express the optative mood (願望法), that is expressing desire for something to be. The modern Japanese equivalent would be ～あったら/あれば いいなあ for ～もが and the first two variants, and だといいなあ for the last three, or alternatively in some contexts ～ますように is possible. It also seems to alternate with a intervening 「に」when it’s coupled with a noun. It’s unclear when exactly its use is existential and when it’s copular, but knowing the general meaning, pick an appropriate translation based on what makes more sense.
Ex) 足（あ）の音せず行かむ駒（こま）もが。（足音を立てないで行くような馬があったらいいなあ。）It would be good if I had a horse that made no footsteps as it goes.
世の中は常にもがもな。 （世界は永遠だといいなあ）It would be good if this world were eternal. [May this world be eternal.] *note the に
The “it would be good if” are translations of the modern Japanese. Translating the original with “may” or the like will sound more poetic as well as distinguish the original.
Classical Japanese 