well, there was only 12,000 VTEC (B16A's) imported into the US over the entire run of the sol's so they're kinda rare. It's a VERY popular swap though and the engines were put into the Civic Si's up to '00 so you can still get them w/ low miles. 160hp USDM
The SOHC VTEC (Si) uses the D16Y engines just like in a civic EX. Don't know how many of these were imported, but i'm fairly sure that there's alot of these (mine included). 127hp USDM
The SOHC (S) made up the D15 engines D16Z(i think, not sure on this one). These were most of the market imports. 103-108hp (???)
There's some people around here that have gone completely banannas and stuck H22's (prelude engines) in their sol's. Thats alot of power. Others on other areas of the board (Civic or CRX forum) have taken a SOHC VTEC to 200+ hp. That's also alot of power.
I guess i'm saying that it's tough to say what the most frequent motor is. The most popular is probably the B16A's due to the 'plug and play' swaps and the reliability of the motor.
just my opinion though.......