you can get 195mm or 205mm section width tires for those wheels. I would do that because 185 is WAY to small... i'd be worried about curbing and whatnot. Is that what came on there stock?
The B16a (Civic Si/Del Sol motor) is different from other civic motors in that it has two overhead cams instead of one. It also has variable valve timing -vtec (which the civic ex also has, but the ex is sohc vtec). The B16a shares more in common with an integra motor than other civic motors.
The RSX-S (K20) motor swap into a 7th gen civic is pretty straight forward, and can be done using OEM parts.
An accord V6 into a civic would be way too much trouble, if its possible at all...
hope that answers some questions... Welcome and enjoy :wavey: