Ok, heres the deal.
You can take this project as far as you want to go couple hundred to a couple thousand, but heres what you can get away with. You can take up to .020" off the heads and use the stock cam and stock pushrods. You'll have issues of valve ticking, so you'll need to tighten up the valve clearance to stop this. It will not backfire, you'll have more compression which equates to more low end torque. You'll have the same lift, but the valve train geometry will be off. Running custom pushrods anytime you adjust head thickness, or deck height will hep things run better, but its not critical at .020".
First thing decide what you want to do with this motor. If you run ea82/ea71 pistons and take .020" off the heads, you may be stuck with running premium fuel to stop detonation, but it will sound like a chainsaw and tear up the road. If you want something a little more than stock for the road, just put ea82/ea71 pistons in and run a stock ea82 efi setup on it. If you want an animal(a little one), take .060 off the heads, run a custom intake, custom pushrods be stuck with aviation fuel and only get 100-130hp.
Add up how much your preferred option costs and compare it to an ej swap. An EJ18 puts out 110hp stock is the easiest EJ to install. Search around and decide what you want. Theres tons of EA81 build threads and tons of EJ18 swap threads.