Absolutely, definitely, your best bang-for-buck is keeping an eye out for a larger A-series engine. Info varies depending on which site you get it from, but for example changing from an A12 to an A15 is around a 30% power increase! You will never get that from an A12 without 'investing' a massive sum of money.

Once you have the bigger engine, you can then look at options like carbs, turbos, whatever you like - and any gains you get there will also be larger than gains you would have gotten by modifying the A12.

Next step in my opinion is to look at an L-series conversion - more complicated than an A-series swap of course, but the results are a whole new level. A12 to L16 is a 55% power increase and L20B is 61%... again all of these figures are before modifications... so you have a nice, happy, unstressed engine that is great to drive and pretty good on fuel compared to something with a big cam and twin carbs.