I have a couple of the answers
to 1. 108 Hp is the stock HP for a 91 Si
2. All civic Si's had tachs. the civic with no emblem, 70 HP and no tach is not a Si. its a base model or STD as we call it.
3. the diff engines you can do are on this page.
http://www.4thgenerationcivic.com . go there and click on the engines link on the side.