Is yea an acceptable replacement for yeah?

Yea is equivalent to yeah, just not in common usage. Yea sounds a bit more suited for a futuristic rather than a contemporary setting.

To assess whether a spelling is considered correct, your best bet is to check a couple of dictionaries. Once the spelling passes into acceptabled use, dictionaries will record it it. I would not use alternative spellings until either: (1) they become standardized, or (2) you are trying to write a specific dialect. Even then, it's better to write dialect using correctly spelled words, but choosing words that reflect that region, rather than misspelling, collapsing a word together, or using other methods.

Two other things: even if a dictionary lists the spelling as correct, remember that the first entry is the most proper entry; and, I would refrain from using a word that can be misconstrued by your readers. For instance, If I read yea in your text, I would read it as yā, either as "Yay, he did it!" or as the older yea and nay, and then wonder why you're including Early Modern English into your text.

'Yea' is a real but antiquated word, still in use in various contexts e.g. 'Whose job is it to say yea or nay?

'Yeah' is the representation of an informal pronunciation of 'yes'. (OED).

So if you are reporting informal speech I would suggest you stick with 'yeah'.