What about STCs to replace TIO-540s with TD450s? Continental 300HP diesels.
Continental: Diesel Line Expanding, STC's Planned
Other articles indicate these engines should be in production by 2015-2017
W W W . D I E S E L A I R . C O M -- The Diesel Air Newsletter -- Home
For lower time PA31-350 airframes (well any with less than say 24,000 TT), this could be an interesting JetA1 burning option, replacing the
evil leaded 100LL, and remaining a significantly cheaper option in maintenance terms than going the turboprop route.