trd does produce turbo kit for bt/svt
go there for more info
http://www.geocities.com/MotorCity/7177/trdturbolevin.html
http://www.4age.net/gallery/engines/025.jpg
normally if wanna play turbo on 4age, normally ppl will use gze and the gb to mod to turbo, coz the drive shaft is "fatter" so can stand more torque, and engine uses lower compression piston, basically is plug n play,
as for 20v, me mechanic tried to play turbo before, but stop half way due to setting problem...
if use gze piston and some setting i think can lah,, budget 3k wor... i dun it is worth it lah...