Have you had the Fuel Injectors looked at? or the Glow Plugs?
It sounds like it might not be buring all the Fuel.
Why would you want an engine from Japan? For sports cars. Yes you want the Japanese Engines. But since you already have the Diesels right there you could easily find something to replace it with.
Check out
www.ih8mud.com HUGE Landcruiser website. There are lots of guys there that can help you decide on what engine to replace it with. If you need to replace it.