Use the Controls.Find() function like this:
Dim searchwhat As String = ini.ReadValue("settings", "searchwhat")
Dim matches() As Control = Me.Controls.Find(searchwhat, True)
If matches.Length > 0 AndAlso TypeOf matches(0) Is RadioButton Then
Dim rb As RadioButton = DirectCast(matches(0), RadioButton)
rb.Checked = True
End If