Bits like the cam you ignition you rather have to buy new as they're likely to be worn in the first place. Your dizzy and what not would no doubt be suitable. But if you're going to go down that route then I wouldn't bother. Really, just sell the thing. I have a 3.9 here which used to be complete + a good runner before I sold the ECU and AFM, but I'd have swapped it for your 4.0 + =A3100 cash a few months ago. You'd win as you'd get an engine of the same capacity with all the bits so you didn't have to mess around and I'd win because I'd get an engine that I could mess around with ;-) Yep, sell it and get a 3.9. Save your effort for the rest of the vehicle.
There's not that many people have done anything with lpg though there are some. It will do fine with LPG, you can easily add dual spark mapping so your ignition will be perfect on petrol and LPG:
Building the thing is not so much of a problem, it's fairly easy and quick, its the installation and tuning! That's where you'll find someone who's done it before can really help out.
Regards
William MacLeod