Do all the usual things full service.
Plugs, leads filters etc.
Carby kit won't go astray given the age and k's,
and may just be the exact problem
Fit pertronic I or II.
keep points for emergency.
Pertronic is a must have for improving engine.
LPG likes a couple of degree more advanced timing.
not to advanced as petrol will suffer and lose of top end etc.
Lpg also likes a smaller plug gap.
Is the auto choke system ok?.
any broken linkages ?.
all vac line ok?.
Take it to lpg specialist for a service/ tune up also.
All these things will improve a Sound engine no end, including economy on either fuel.
This is in a nut shell so to speak you may want to research for tuning at home further.
cheers