I have read that the model of the Romulan Bird-of-Prey was actually a very quickly-designed model which was partially based on the Constitution-Class saucer, and nacelles (Which is one of the reasons why it was written into the plot that the Klingons and Romulans were working together, and they just used the Klingon D-7's as Romulan ships)
Umm, this seems to confuse two issues.
In "Balance of Terror", the original script spoke of Romulans using industrial espionage to design their ship. Hence, Wah Chang was told to create a Romulan ship that looked a bit like the Enterprise
, so that Stiles' paranoia about spies would gain some credibility. The final script dropped the industrial espionage angle, but the Romulan design still retained its Federation looks.
In "The Enterprise
Incident", the Klingon ship model was used for Romulan ships. Two reasons have been quoted: either the original Romulan model was lost (misplaced, broken, or stolen and/or destroyed by Chang who felt ill compensated for his work), or then the producers wanted to showcase the expensive Klingon model instead of the cheaper and older design. This prompted the script editors to insert the bit about Klingon-Romulan cooperation. Or in fact, the bit about Romulans using ships of Klingon design, which may mean anything from cooperation to copying to outright theft.
Wah Chang's Romulan ship doesn't actually reuse any Enterprise
molds, so rapid construction wasn't the underlying reason behind the design.
It's a separate issue whether the Romulan ship was supposed to have warp drive. And certainly nothing in the episode indicates that the Romulans as a species would be unfamiliar with warp: their weapons can outwarp the best Starfleet starships, after all!