As long as you haven't started the engine or turned the crank to where valves have struck pistons and bent, then its just a fix.
Someone else will have to say whether the headgaskets need to be replaced again. As for the cam timing, all that's required is to pull the timing belt covers again, and line all of the marks for the crank gear and the cam gears with their appropriate case markings, slowly release the belt tensioner, test it for proper clearance, and test rotate the crank shaft to check for parts banging together through several rotations.
I'll not go through all of that here, as there are videos on YouTube (view the proper ones for an EJ25) as well as downloadable factory service manual pages that will take you step by step through it. Good Luck!
If you haven't got the cams in, all the valves should be closed. So just line the crank mark up correctly, as the pistons are half way up the bore, the put your cams and sprockets on and line up the cams.
You obviously don't know what you are doing. PLEASE seek help before you go any further, or you will do damage to your engine.
There are videos on YouTube for your engine, showing how the timing marks are placed on each cam gear, as well as the crank shaft sprocket. If you have all of those markings lined up correctly, and the timing belt is correctly placed on its routing around the gears, and there is the proper tension on the tensioner assembly, then you've done it correctly. Forget about the pistons. If the marks are all located correctly as I described, the pistons will be in the proper position as well.
I've added a PDF file from the factory service manual, showing the step by step timing belt replacement for the SOHC engine. Read it carefully 2 or 3 times before you try it. If you get the belt one tooth out anywhere in the process, the engine will not run correctly.
Get educated on this process and be sure you know what you're doing before you dive in. Get it wrong and you could be in for a lot of grief and expense. The job is a DIYer if you take the time to study it first, and then check to be sure you did it correctly before you crank the engine.
You will not damage the valves or the gaskets if you just do everything in order.
No cams on the heads, turn the crankshaft to line up the timing mark at 12:00.
Install the cams onto the head with the timing marks on the gears at 12:00, being sure the torque and spacers are correct for each cam cap and lobe. You can temporarily place the gears on the end of the cams to line up the marks properly. You can take it back off the cam once this is done to attach the rear cover.
Don't sweat it. Take your time. Its not as hard as you may be making it out to be.
Once the cams are in, new seals, the rear covers are installed and you have the new water pump in, then install the belt. After you have the belt installed, then you can go back and check the valve lifter spacing since doing this requires rotating the engine.