These watercooled engines are a fortune compared to their aircraft counterparts, i had no idea there was such a huge difference in price 
well, maybe i could use the aircraft engines then, just dont tell them; or they'll nag me/warn me to buy the more expensive marine versions, but i need to check out carefully the aircraft engine's specs.
the only main difference that i can think off will be the lack of cooling, but there might be something else too, but you can make your own watercooling easily anyway, just bend a thin tube around the case 6 times, like the Graupner version.
i'll investigate these aircraft engines this weekend and if they're ok, i'll buy them on the quiet!
to be honest, i'm reading a lot of BULL*** with regards to these hi- po marine engines on other forums, it's all about buying ``bling``and this ``bling`` costs a fortune, because i'm sure that you can also use large scale ``Buggy or Helicopter`` engines as well, but you need a decent shaft diameter, between 5 and 8mm.
i bet all these monster engines are the same, just dressed differently.