To be honest, Willkie would likely govern much as FDR did, but without his experience and political skills.
Willkie was not an isolationist. He was an internationalist, and he became the GOP nominee because after the Fall of France, even the GOP voters wanted someone who would help the Allies short of war and they rejected Taft's isolationist foreign policy.
Both Willkie and FDR catered to the domestic opinion which was still against US entry in the war. Documentaries like to show Willkie attacking FDR's policies (The World at War, in one of its few categorical errors called Wilkie an "out and out isolationist" which is completely false), but both candidates said the same thing which was "help the Allies short of war." The only difference between them was that FDR said, "I'm not going to send our boys off to fight into a foreign war," and Willkie said, "You can't trust FDR not to do that," and FDR would say, "No, you can really trust me."
If Willkie had won though, he'd probably do more or less what FDR did. Would he do Lend Lease? Tough question. FDR only did it once it became apparent that Britain couldn't continue to pay. Willkie will be faced with the same situation. Willkie, like FDR, will still want to help the Allies, and he will be faced with the challenge of how to do so. We know FDR's solution. It's impossible to say if Willkie would have come up with a near-identical solution, a workable alternative solution, a failed alternative solution, or couldn't do anything at all. However, the US has a history of ex-Presidents giving new Presidents advice. It's possible FDR would see the problem and give Willkie on what he would do if he was still President. So ultimately I think something would happen.
It's important to note that in reality, Willkie was an outspoken defender of Lend Lease, and that FDR used him as an informal ambassador at large throughout WWII (very much unlike FDR's handling of Herbert Hoover).
So much of the year of 1941 depends on choices that FDR made, and the Axis reaction against that, that we don't know how those critical choices will be made. Would a Lend Lease style program be extended to the Soviet Union? Perhaps, but if so, not to the extent that it was under FDR. Willkie and the GOP might bargain a lot better to get Soviet concessions. Would he put in an oil embargo against Japan? Probably something, although not a complete embargo. Would he move the fleet from San Diego to Pearl Harbor (which was done in opposition to most of the Navy)? Probably not. Would he make as wise picks as FDR as to the top brass of the Navy and Army? Probably not, but the best would likely still rise to the top.