The 90-94 turbo motor will swap into any 90-94 DSM body, as will the 2.0 non-turbo 4G63 motor. Keep in mind that 90-94 4G63's come in two variants, referred to by their flywheel bolt pattern: six bolt or seven bolt. It helps to swap a motor from the same model year.
The 90-94 motors can also swap into the 95-99 cars, but differences in the motor mounts, front cover, oil pan, and more make it difficult. Magnus Motorsports has a nice online FAQ file about doing it. The whole reason behind doing it is that the earlier motors don't seem to suffer from the dreaded "Crank Walk" problem, and the 90-92 blocks have particularly stout bottom ends that lend themselves well to huge power increaes.
The 95-99 turbo motor has a different motor mount configuration that cannot bolt straight into an earlier car. Although it could be done with significant work, we don't know why you would.
Galant VR-4 motors (from the U.S. or Japan) can bolt fairly easily into the 90-94 Eclipse, or the 95-99 with more difficulty (see previous paragraph). The "Cyclone" dual-runner intake manifold is found on lots of the Japanese motors. It's not known for much increase in peak power, and U.S. ECU's lack the facility for controlling it, so it's not worth getting over a decent domestic motor. On the other hand, the Japanese motors can be had for pretty low prices, so if it comes with the fancy Cyclone manifold, that's fine.
Lancer Evolution I-III motors SHOULD bolt pretty easily into 95-99 Eclipses, but they're ridiculously expensive, so we don't hear about them much. We haven't seen it done.
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