As you guess, the engine would be either Kinsei or Zuisei.
Cooling fin design has nothing to do with the designation.
As another clue, stroke 150mm was for Kinsei as well as 130mm for Zuisei.
If the stroke should be 160 or 170mm, your relic would be a very historical model "Shinten" as this was almost experimental model with production quantity only 119 sets. Mitsubishi A20 experimental engine (18 cylinder type of Kinsei) was also designed based on the bore/stroke 140/150mm for your reference.