Well...there's a "2.0 Si" and an "Si". The 2.0 Si obviously came with a 2L engine, and the Si came with a 2.1L engine. But if it was replaced, then none of that may even matter at all. You should look for the engine code on the front of the block. It's a flat plate where the engine meets the transmission. Like here:
This one is a '92-'96 JDM H22A Prelude engine. If it was a USDM engine, there would be a number after the A (like H22A1). And the first 2 numbers in the engine code represent the displacement. So with the H22A, it's a 2.2L. The B20A5, from the 2.0 Si, is a 2.0L. And the B21A1, from the other Si, is a 2.1L. I honestly have no clue what the JDM counterpart for the '91 Prelude is.