The high temps and lots of gas will slow you down from the previous time. On an automatic launch, brake it, and push the throttle up to the point where it is almost going to move. Then as the the lights fall, push the throttle in as you let off the brake. Push the throttle in as much as you can without letting the wheels spin as you go, but do not let the rpms fall off from where they were when you had the brake engaged. If you were to install a shift kit or a lentech valvebody, or get a tune that firms up and changes the WOT shift points, that would also help a lot on your racing. Otherwise you are looking at getting a higher stall torque converter to let you stall the car up further on your launch.
The shift kit is something you CAN do, just might want to be very careful about it. The lentech valve body will basically be the same as adding a shift kit, but you will just replace your entire valve body assembly and not take your valve body apart and modify it as you would do with a shift kit install.
As far as the engine swap, if you post pics of the engine or email them to me, I can help you a lot. 04 Zx3s were either A) 2.0L zetec engines (exhaust manifold in the front, intake in the back) or b) 2.3L duratec engine (black loopy intake in the front, exhaust manifold in the back). If your engine looks like mine in my nitrous thread (with maybe a dull gray valve cover though), it is a 2.3L, likely a PZEV if it has the dull gray valve cover, considering the year.
2005 Focus Zx3 se D20 M5 - "Helios" - blazing copper
intake, header, exhaust, xcal2 tune, !CMC, cams, suspension, nitrous, torsen, spec lw flywheel and stg1 clutch
home brew turbo in the works