Couple other things -
Using Silverstreak's hood/chest would allow you to completely bypass painting over the Nissan logo. Using Silverstreak's roof would bypass the need to remove Prowl's sirens, although we can't see it in the picture to be certain.
Going through this excercise makes me want to waste money on doing this custom myself (well, minus the painting - that I'd farm out...I suck at painting). It would be expensive, though. Still step by step, this is what I would do -
1. Acquire another copy of Prowl/Silverstreak...no KOs. Order the Bluestreak sticker label sheet.
2. Hunt down the blue and gold paints to do the body and "horns" on Prowl's head.
3. Start taking them apart for reverse-engineering to put them back together.
4. Take the following parts from each:
Prowl - Head removing the face, Crotch, midsection, black 'neck' part because Silverstreak's is red, legs.
Silverstreak - Everything else.
5. I'd attempt to put Silverstreak's face in Prowl's helmet. If it doesn't work, I'd probably not go any further.

6. Assuming 5 is a go (and I think it would be), I'd disassemble the head again, because there is no need to paint the face, start the chassis/arms/hands/horns paint process (hiring somebody to do it).
7. Reassemble my box accurate Bluestreak.
8. Apply Bluestreak labels.
9. Photograph the finished product. Upload into this thread.