These engines are known to eat cam lobes if they don't get fresh oil often.
A quick ish check is use a dial gauge to measure lift on each rocker by removing valve covers and turn the engine by hand.
It's not super accurate but you will see if one or more rockers are hardly lifting at all.
Then it's new cam/lifters/chain/sprockets...
Plenty of early TVR owners are happy with the "flapper" injection, (when it is all working properly)
So, go through the full diagnostics and replace any ancient sensors that are failing.
This is the cheapest/quickest option.
Bin the whole LPG system.....ðĪŪ