Any petrol engine you fit is going to use a fair bit of fuel - the ford 2.9 cosworth you mention was a very thirsty lump...
These are not aerodynamic vehicles, and diesels are often chosen because of their economy.
LPG is worth considering if you want power and economy - I run a 4.6 RRC auto, and get 16-18 mpg, which equates to a 32-36mpg petrol running cost.
Drivability is very important too - a broad spread of torque is far more useful than an ultimately high BHP - do you want to be hunting through the gearbox to keep the engine spinning all the time? Another reason for the favouring of the turbo diesels.
With the Defenders, anything is possible, from the sublime tuned 200 BHP TD5's to the ridiculous JE Engineering supercharged monsters - take your pick.
Power costs money - how fast do you want to go?
Any of the existing pre-fitted Landrover engine solutions will be relatively easy...
The 300tdi can be tuned for more power - have a chat with the tuners.
TD5? 200bhp is available, but you might prefer to just go and buy a TD5 vehicle...
A decently tuned 4.6 V8 will go nicely, make ALL the right noises and can be fitted at reasonable cost ( but will cost a lot to run if you don't convert it to LPG).
The T-series (disco mpi) engines are good, if revvy, but a turbo from a 620 or 820 can be fitted.
If you look at changing to a "non-90" engine, there will be complications to consider, especially with electronic fuel injection etc,.
A BMW 2.8 straight six engine can be fitted - I always wonder what this would be like to drive - probably quite nice.
Cummins - never driven one, so can't comment...
A jaguar engine could be fitted? ( the 4.0 V8 is excellent, and available with a supercharger )
Saab Turbo? ( these are one of the cheapest and reliable performance engines out there, IMHO - an easy 200bhp engine that will do 250k miles is serviced properly)
Lexus LS400 engine and box has been fitted by some - you can get a conversion kit to fit the LT230 transfer box onto the back of the Lexus transmission - 280 BHP from memory, and absolutely bulletproof mechanicals.
Take your pick... The sky is the limit.
The farther down this road you go, the more money you will spend getting it fitted, and the knowledge base of people to help out if it goes wrong gets thinner... Also, the resale value isn't always improved - your engine choice is not going to be universally liked by all...
Also, as you go to the bigger power outputs, the transmission will need upgrading, as well as probably brakes, and suspension ( especially if they are already tired) - all adding to the cost.
I will probably get booed off the stage for saying this...
I have a hankering to fit one of the Ford 2.0 or 2.3 "DOHC" engines to a 90.
These were fitted to late sierras, granadas and transits, replacing the old Pinto engine - they developed a reasonable 115 to 150bhp, depending on spec, but were real stump pullers, developing torque at very low revs ( peak torque at something like 2000 revs I think) and were the last of the "lean burn" technologies that were made by ford - being killed off by the catalyst rulings, so were excellent on fuel due to the high CR.
In use, they were very relaxed torquey engines, and the later defenders had ford transit engines, so why not?
You have to ask yourself what you really want, and how much you want to spend.
You might find that it will be cheaper to upgrade the vehicle to (say) a TD5 than get a TD5 engine fitted....
If all you want is a better vehicle for the motorway, then perhaps an engine change isn't really what you need.
Changing the transfer box ratio ( or wheel sizes) can lengthen the gearing to make 70mph a lot more bearable.
You can freshen up the 300 tid with a new intercooler and some retuning, fit a soundproofing kit, and you have a nicer vehicle all round, without needing to shell out a huge amount of money.
Work out what you really want first... But remember - it will always be a Landrover...