I wouldnt modify a D series motor. They can make good power for cheap, but thats if you turbocharge them and then they have terrible lag b/c the head doesnt flow very well.

if you decide to do a motor swap atleast get a bigger motor than what you already have, which means more than a 1.6 liter. Then its on what you can afford. I like B20's over LS motors. But I would go with a GSR over a B20 or LS, then you have the Type-R motor which is also nice but expensive.

I would not go with a H series motor, but if you can afford it K series is nice but if you decide to go that route get atleast a rsx type-s or type-r motor.

These are just my opinions with how i would go.