Certified or Experimental?
The most common conversion available will be the Centurion, either 135hp or 155hp from now Continental (ex Thielert - conversion of Mercedes Benz A-class engine for aviation use). Disadvantage of these engines - there is no TBO, only TBR (no overhaul options, only replacement, and the 155hp has only 1200 hours TBR) and not everybody is amused by FADEC flying. If the N is Experimental, conversion to VW Diesel engines may be also an option.