It might be worth a call to Davis diesel he might have one he sometimes heads not listed on his sheets or web site martin
also enya has released the 11,15 and 25 as diesels very nice but not cheap about $139 from modelflight AUS hobbsy and I have the 25
He (davis} does have heads for the OS and Fox 15 if you want to stay in that size, the OS LA series make great conversions 40, 46 and the engine prices are not too bad TTS and STs also any thought is PAW lots of choices an prices not bad