What are best / cheapest ways to increase power from the stock A12 engine ? As well as being cheap also.

Best bet is to buy an already modified A12.

To do it on the cheap with biggest bang for buck, turbo charge it.

Otherwise:

Electronic ignition
Downdraft weber
Extractors
Higher Comp Ratio
Lightened flywheel

You can do all of those modifications on a small budget or a large budget.

'Cheap' would need to be quantified to offer accurate advice. How much do you want to spend?

To increase power for $350- $500 and retain an original look install an A14.

Looking to spend under $800 as I'm still at school with only a casual job. Will I find a good A14 with for that money?

Absolutely. Running, good condition A14 engines will cost anywhere between 350 and 500. Anything higher than that is in my opinion, expensive.

Unless it is a proven recently reconditioned engine.

Proven means receipts and documentation from a reputed workshop. Not just repco receipts for pistons, gaskets, etc.

Thanks mate I'll look into, would a new exsaust say 2 1/2 inch help on the A14?

Do I need to put/ modify anything to get the a14 in?

Just need to show us which engine mount brackets you have installed. The A14 takes a different bracket on some A12s otherwise should swap right in. Also look at getting a 32/36 Weber downdraft carburetor. Its an affordable upgrade from the standard carb and gives more power but less fuel economy. All you need is an adapter plate and it mounts to the standard intake manifold. Can be bought online.

2.5 inch would be huge and I do not think there would be any power gains.