[CNGBiFuel]

I'm afraid, Classic has it right.

You can use the factory fit CNG fuel-gauge for sure. It takes a 0-5V input. Better than me will say which LPG equiv. sensor does that, it's pretty standard. Your installer will duplicate the output of the CNG pressure sensor. I've flipped two cars from CNG to LPG and vice versa. In each case you'll retain the factory-fit gauge.

You'll likely end with two LPG gauges, one as now (former CNG) and one wherever the new switch gets plonked. These systems don't have to be installed in ways to look unsightly. Sadly, usually they are.

My changeover switch is on the transmission tunnel. It looks like it was meant to be there. The trouble being, this part of the install took a day in itself. That's a labour charge a non-DIY installation likely wouldn't be prepared to find. Using the CNG filler/ opening / bracketry will potentially make a very tidy factory-fit 'look', but there's a good day's work again. This against firing a drill thru' a panel in 10 mins.

Classic will be better informed, can't see why your installer can't keep the principal stainless pipe runs. Mate the LPG fittings 'top & tail', to it. It'll stay factory in 'look'. Careful removal of the CNG receptacle for its LPG match. More work. Bigger bill.

Make sure you keep your Necam parts. Stepper and reducer. Very sought after.