The boxer diesel isnt lower than any other automotive diesel (they have mooved all the crap up on top of it)
how much room do you have 18 or 20 inches, and how much room above crank level??
it would appear that the best way of doung it would be to either find an automotive diesel and perpaps shortening the oilpan or if you really really needed it to be 18 inches tall, buying an inline 4 or 6 and laying it on its side, but thar would require modifying the oiling to dry sump which costs a grand or two depending on parts and your skills
a Jonn Deere engine would propably be well suited as you could add scavenging ports to all the sheetmetal covers on the left side of the block and plumb them to the scavenging pump
a dry sump would also facilitate oilchanges
or how about this one: for that price a couple of grand at the local racing engine builder flipping stuff 90 degrees and building a dry sump oiling system seems within reason http://cgi.ebay.com/ebaymotors/Cummi...Q5fAccessories