My father does not pray and is considered sinful (fasiq). Is he valid as my marriage guardian?

In the practice of Malaysia's religious authorities, a father who is still Muslim is generally still accepted as guardian even if lax in practice, since the view requiring an upright guardian is often eased to avoid hardship; but a father who has left Islam cannot be a guardian.
The Prophet ﷺ said there is no marriage except with a guardian (Sunan Abu Dawud 2085). In the Shafi'i school, the conditions for a guardian include being Muslim, adult, sane, male and upright; later scholars of the school discuss that a guardian's sinfulness does not cancel his guardianship when many people are lax, so that marriages are not held up. If in doubt, tell the religious office or officiant before the contract; they will decide whether he can give you in marriage or a judge-appointed guardian is needed. Use this as a chance to approach your father kindly, not to judge him. See Marriage guardian.