local will be no problem. on a national level you might run into problems though. although if you looked at past years national results, 88-95 civic hatches always seem to do really well in SM. for STS or DSP i see a 99-00 si being a bit on the heavy side with not enough tq. to really be competetitive. again this is on a national level and if you are only concerned with local and having fun(which its all about having fun anyway) i say go for it.

i built my car into a class that id stand no chance at all(DMOD) on a national level, but im still able to be competitive in my local clubs. it wont bother me because i built my car to have fun with, not for the intention of winning a trophy. thats what to keep in mind