In hilchos teshuva of the Rambam (4:1), he states that one for proselytizes for idol worship is unable to repent. Are there any other sources that comment on anyway to repent for such a sin?
The Rambam doesn't say that such a person is unable to repent, just that Hashem won't provide him an opportunity like He does for most people (אין מספיקין בידו לעשות תשובה). In halacha 6 there, the Rambam clearly states that neither this nor anything else prevents (מונעין) teshuvah, just impedes it (מעכבין).
Coming from a more reformed opinion, I think Hashem would have no issues with your repentance. It says in Yechezkel 18:21-23
And if the wicked man repent of all his sins that he has committed and keeps all My laws and executes justice and righteousness, he shall surely live, he shall not die. All his transgressions that he has committed shall not be remembered regarding him: through his righteousness that he has done he shall live. Do I desire the death of the wicked? says the Lord God. Is it not rather in his repenting of his ways that he may live?