You could just use a SCR voltage regulator for the lamps.

How would that work & what wattage would it run please?

SCR voltage regulator is basically a dimmer set permanently to one level. The level is chosen so, that heating elements when dimmed to this level from 230V will dissipate the same power as when running at full power with 120V. This method is only suitable for driving heating elements. It is not suitable for any heating appliance with a fan or built in electronics either

It cannot be used with any lamp type :

Anything electronic will be destroyed by the peak voltage that's higher with such "dimmer" than with the correct 120V supply, and by the inrush current to capacitors on each half cycle

Anything magnetic won't work correctly with a non sine wave supply, and may overheat as well

Incandescent filaments have very small thermal mass, in some lamps it is small enough that the filament heats/cools significantly throughout the AC cycle. It will reach higher peak temperature with the "dimmer" than with proper 120V at the voltage peak, so burn out. The higher voltage peak will also increase the risk of a flashover inside the lamp

If the SCR fails, it may pass through 230V as is, or it may partially fail and pass a rectified half wave, which will drive magnetic devices into saturation and high current draw, which will destroy them if it continues for a while

So I’m better with the transformer?

Yep absolutely, just know what is its power limit

SRC voltage regulars "chop" up the waveform don't they? My SCR voltage regular works fine for universal motors, it also worked fine on a heater.

The guy I got this transformer from said it was safe upto 500 watts which covers the majority of lamps I have.

With transformer loading its the current (so, apparent power - VA) that counts and not the active power (W)

As example, 400W SON lamp will take 440W with the ballast. If connected without the capacitor, it will draw 1060VA, a 2x+ overload for this transformer. With a properly working, correctly sized capacitor, it's 480VA. (Numbers in this example are actually for 230V SON gear, but the concept is the same with 120V)

By its size looks like it'd be at its limits for 500W, unless it is an autotransformer (which is ~half the size for the same power)

The power it can handle depends also on the running time. At load levels from no load up to moderate overloads, the limiting factor is heat. You can overload it to an extent for short time (shorter than it takes to heat up the windings), or run at 500W for a while, but for continuous running for a long time, load it to less than the max 500W

It will be used for testing only so short time on incandescent lamps so shouldn’t be any problem, the previous owner is highly knowledgable in this field so it must be ok