Before you go looking at performance improvements, try going for the cheapest & best performance tricks. Give it a FULL tune up.
Check the compression. Thats always a good start. If your engine is down on compression then fitting twin carbs Etc. will not help much. Get the tappets correctly set, replace the points & maybee even the condenser. Set the dwell angle correctly & monitor it at about 1,000k, then 5,000 & 10,000k. Set the timing correctly & make sure that the vacuum can works correctly & does not leak. Replace ALL filters & fill with the correct grade of oil for the season & the use that you have for the vehicle. Make sure that the brakes are correctly adjusted & are NOT dragging. Make sure that the carb is correctly adjusted.

Depending on just how good or bad your car is tuned & serviced, you may find that this perks it up quite nicely. This is the baseline upon which you base further improvements, & this is abiut as low cost as you can get.

If the compression is down, then rectify the CAUSE of the problem. This usually involves the good old "ring & valve" job & is recomended before adding any performance improvements to a "used" engine.
Chris