what did you use for tuning ?the spi crank has 4 extra counterweights and is way way way way more balanced than the 1.9. and by way more balanced i mean, its balanced period. the 1.9 crank is just cast and thrown in. buying new pistons is way more work and cost than its worth. the flow characteristics of the spi with its dome top pistons is perfect, especialy when the intake is slightly polished up. but not totaly polished up. the roughness helps atomize the fuel because of the randomness of the direction of flow. the spi is a verry strong motor stock. mine had about 55,000 on it and i boosted 17psi with arround 9.5-9.7:1 for 2 years before i decided to rebuild it for longevity purposes.
if anything, i would shave the head about 5 thousandths and call it good. the xtra compresion will give you better numbers all across the board.