I cannot think of any other agencies that could perform the function of the IRS. Many agencies have efficiency problems, but the IRS is generally regarded as one of the more successful ones if one can believe that.
Or are we talking about repealing income tax? Honestly, that could hypothetically be done. But it would mean completely cutting all defense spending, defense-related spending, foreign military adventures, foreign-policy-related spending, para-military and espionage spending, a significant amount of federal law enforcement spending; it would mean legalizing all drugs and just about everything else neocons would never want to do. Then, austerity would have to take the form of higher-level bureaucracy (does the President need a personal staff of 2,000 people?, Does the Secretary of State need to travel to foreign countries when we have ambassadors (do we need so many ambassadors?)?, Do we need dozens of under-Secretaries and Deputy Secretaries of Defense?, can DHS' agencies do well without the bureaucracy of the DHS (est. 2002) over them?, etc.). The civilian agencies that keep the roads repaired, float school loans, operate national parks, regulate food and drugs and everything else might be able to survive from funding from other sources of revenue other than a direct income tax. In other words, the income tax basically funds the global American Empire. Of course, there is the interest on the public debt to consider as well. Then, do we want to go further and cut Social Security and Medicare?--both of which are not actually federal income, but are trust funds for ourselves that we put into all our lives (media and politicians call them "entitlements" for some reason). But all of the other national security/police state cuts mentioned above could not really happen either. We more or less "crossed the Rubicon" when we became the caretakers of 10,000 nuclear weapons. This sort of reorganization could never be done by a pluralistic government like ours.