In fact the initial version of WPF ran on .net 3.0 so in principle that's the minimum version that you need. Of course, .net 3.0 requires XP or later and does not support Windows 98.
The essential problem that you have is not that the framework installer won't run on Windows 98. The problem is that .net 3.0 and the WPF framework use functionality that does not exist on Windows 98. Even if you get all the .net framework DLLs onto Windows 98, they won't run because the depend on functionality that does not exist on Windows 98.