The body panels are different...the HF has a thin stripe down the side, and the SI has a thick stripe...the side moulding.
Engine wise......everything is interchangable...and most of the people that wants speed will drop in an 86-89 integra D16A1....I went homemade turbo...and I have more fun than they do....
Reason...the engine is light and compared to the car...it's light. A turbo would only add 20-50 lbs at the most....but also more power, rigging problems and tuning...done right...it kicks ass.
The other side is the 100lbs+ engine going on a car that was not ment to hold up to it, the cornering ability is lost, and only straights are fun. You will have to swap out the torsion bar for a heaveier one, and other shit like the hubs, axles...etc....
plus the brakes will not stop a heaveier car like they do the lighter car...unless you upgrade those too.
Other rumors about the cars are the HF had the roller rockers and you would need to rape another head to make the SI fully roller rocker.....8vavle to a 12 valve....simple math.
More in depth info can be found
here...