He was reckoned to be on the more conservative wing of the party, so there won't be anything really radical, but he will probably support measures like the Freedman's Bureau and Civil Rights Acts, which Johnson vetoed.
Foster would have been succeeded in 1866 by a new President elected in November 1865, probably a Radical Republican.
So the course of Reconstruction would be substantially altered.
The successor would almost certainly have been General Grant, who wasn't as close to the Radicals in 1866 as he became later.
There will be some change to Reconstruction, but probably only in the short term. The basic problem was that most northern voters did not care deeply about promoting the rights on negroes, while southern ones
did care about limiting them. That difficulty remains, whoever is POTUS.