http://www.lexis.hi.is/beygingarlysing/index.html
Modern Icelandic paradigms. Type the "dictionary form" in the top
window (e.g. nominative sg. for a noun), or any form in the bottom
window. I don´t know if there´s been much change in the dat.sg. of
masculine i-stems. The few I checked just now seemed the same. I
know that a few old a-stems have acquired i-stem like plurals over the
years...