Well If what Iv heard is right the USDM '90 teg already had a B series engine in it?
If so then its a very easy swap. Literaly bolt in the B16a useing the same mounts g/box etc. (or you can swap to the JDM g.box some of which have better ratios for the heavier teg. (you want an S1 or YS1, if you get a Y1 use your orginal box, and sell the Y1 for a rediculas amount of money)
Then all you have to do is some minnor wiring for the VTEC.
IF however the USDM Tegs came with a D series engine, then you need a mount kit and some more wiring. It is however exactly the same as doing the swap in a Civic.