Should a romanian folder drop in to a norinco nhm-90 without major issues? I just wanted to check before I buy. What other folder would be a possibility besides the ace which is a bit modern for my taste?

No.  The receiver is thicker than European.  You will have to thin the portion of the folder that fits in the reciever.  The screw holes are not going to line up with the tang holes and the ones in the stock.  You will only get one hole to line up.

damn. Is it possible to find a chinese folder that costs less tan the rifle. What about the 74 triangle?

No.  Chinese use receiver only, no rear "trunnion".  Rommy trunnion will not fit due to Chinese receiver being 1.5mm vs Rommy 1.0 mm thick.  From there it goes to major issues.  

Like MAK said only one hole will line up.  But if you get the Romainian (sp?) kind that has the push button you could file enough off each side of the block that goes into the receiver for it to work.  I did that for my Polytech postban.  The E. German  folder with the lever will not work because you would have to file too much off the frame like thingy that goes into the receiver.  The pushbutton one works because you are filing a solid block of steel.