98 GS300 won't start.
 

Turned key and car started but idled rough. As soon as I touched the throttle, it died. Checked for spark - OK.
Replaced fuel pump. Still wouldn't start. Came across a
Complete fuel assembly and swaped it over. Now she started straight away. Am I correct in assuming that the fuel regulator that is part of the assembly is faulty??

Guessing is one way to fix your car, it just cost more. Hey but you end up with new parts!

I would try replacing the engine, you'll have most everything covered...

plug in the obd2 diagnostic and see what code its giving you...

Cars need fuel, air and spark to run. If you are missing any of them, it won't. Which don't you have? Its hard to not have any air. So we will forgo that.

Spray Gumout in throttle body. Crank it. If it fires up and dies, then you have spark and no fuel. You can keep it running by spraying shots of Gumout in the throttle body to keep it running.

Unplugging the MAF will trigger a MAF fail, but it will drop to Speed Density mode. Then it will run without a MAF.

Injectors are often all connected to one fuse. So check fuses. I have never seen an injector fuse blow, but none the less, it IS a possibility.

I did replace the whole fuel pump assembly. It fire up first turn of the key. The only thing on the assembly apart from fuel pump is a fuel filter ,a fuel gauge float and the regulator. I am curious to find out what the fault is. But how to check the regulator. I connect power to the top of the assembly but no go. Bought a new regulator and will try that. Let you know if that is the fault.
PS . If I took it to a Lexus dealer it would have costed the same as a new engine!!!!

You hook up a fuel pressure gauge and measure the fuel pressure. Normally you do this before you remove the part from the car.