probably not practical since a GTS is only going to be a few thousand difference from a gt. so unless you've already got a shitload of stuff invested into this car like custom paint and moulded body kit, custom uplostury and stereo painted dash, etc.. I'd sell your GT and get a GTS. now if you are going to do all the work yourself it might be an interesting learning experience and fun to do just for fun.. the engine I heard is 1800 dollars and you can probably get it from a toyota dealership or an autoparts store for that without having to pay shipping but you'd have to pay tax. Monkeywrench.com has built blocks with different upgrades might want to look into something like that. but you could probably get more horsepower for a cheaper price by turboing your existing GT motor with a c2power.com turbo kit for about 3700 since you'd probably need to buy the tranny with the GTS motor as well for another 2 grand. and with that turbo kit you'd be running a lot more HP than a stock GTS motor.