there are plenty of companies out there catering to nissans and especially silvias.. you'll find em. look @ our banners here on zilvia and take a look @ our links. As for sites i can vouch for.
pdm-racing- don is great guy...240entusiast makes plenty of 240 parts and go fast goodies
j-spec.com talk to sam they got mad jdm parts
you can also try wayne @
phase2motortrend
and
autoimaging.com
as far as engines, talk to:
unstablehybrids
and
yuri over @ heavy throttle
btw keep lookgin for that special coupe. you'll find it
hope this helps