Symbol Roof Window

Good evening, bet Im doing this wrong. My first attempt!

My 55 plate Symbol has no roof window left except the hinges (4off) due to the previous owner having an "accident". 
Can I replace the existing unit with a Heki 2 please? There seems many more available S/H. I think the aperture in the roof is the same size but with 3 hinges and gas struts rather than the cable and rack winder system of the Heki 3+. 

Are there any downsides to doing this? 

A complete Heki 2 is cheaper than just a new 3+ window. I guess removing the old unit wont be easy.

All comments welcome. Thank you.