the neon never came with the 420A, contrary to popular opion, it came with the S4RE(SOHC) and D4RE(DOHC) 2.0L engines(and the SRT4 has the 2.4L KZE, but thats completely beside the point), both the SOHC and DOHC engines share the same block with the 420A, but the 420A head is reversed from the D4RE(IE the exhaust is on the front of the 420A where its on the back of the D4RE) now the 420A and D4RE engines cam swap out some parts, like the head, cams, and while a 420A engine will fit in a neon and run on its computer, a custom exhaust setup would need to be fabricated/adapted to the neon(and vice versa for anyone wanting to swap a D4RE into an eclipse RS, the D4RE btw were rated at 150HP rather than the eclipse's 140, mainly due to the D4RE being slightly overrated and having slightly more aggresive cams) anyway, the point of all this is...buy a neon engine, it will fit much easier than an eclipse one