hrm - i dont think u have to keep the D15 intake manifold - im using a D16z6 intake manifold on my D15b7 and as far as i know the d16z6 and D16y4 have the same intake manifold
as for the exhaust - the sensor should be plug and play with the D16y4 - you will have to use the D15 dizzy

if you can afford it now would be a good time to invest in some N*Power headers on ebay (only $160 shipped) and a hondata intake manifold gasket as well - the stock gasket will be complete shite by now and will need to be removed carefully with a blade and some gasket remover spray - install the hondata with a light application of permatex aviation form-a-gasket on both sides
i would also give all the ports on the head, the intake manifold and the throttlebody a good spray with throttle body cleaner

i would probably do a charcoal canister delete at this point as well - if its never been replaced than its doing nothing anway - just plug up the vacc lines and leave the fuel line open for breathing

u should be able to run the engine on an OBD1 ECU - just have to find the closest basemap - im using a P28 on my D15b7 even tho the engine doesnt have vtec - i got it chipped to remove the no-vtec CEL (i also got launch control, and CEL shift light added to the chip as well)