Can I use parts off 88-91 Civic Si for my 87 Si (taillights, guage cluster, seats, etc.)?

 

No. None of the body parts will work. Neither will any interior pieces. You could put the seats in, but of course youd have to mod them. That wont bolt in. The best options for seats in a 84-87 is to get seats out af a 86-89 integra. Theyll bolt in and theyre a lot nicer than the stock Si seats.