[ὁ ἀνὴρ] ὃς ταῦτα ποιεῖ καλός ἐστιν.
[ἀνὴρ] ὅστις ταῦτα ποιεῖ καλός ἐστιν.
Are both these sentences, when converted to definite participle, rendered as this:
ὁ ταῦτα ποιῶν καλός ἐστιν.
The only way to know for sure which of the two is represented by the definite part.
would be if there was a negation, in which case the definite will be negated by οὐ
and the indefinite by μή, right?
Exactly. Just like how the definite article in general can either be specific or generic.
Also, I never quite know when the ἄν is kept when converting to participles,
and when it is not:
subj. + ἄν -> is not kept
opt. + ἄν -> is kept?
ind. + ἄν -> must be kept, right?
I think that's right. I think of it the other way that a participle with ἄν must represent either a (past) indicative + ἄν or an optative + ἄν.