Your tire guy would look at you really funny if you asked him to install 15" tires on a 14" wheel
Seriously, you won't find many tires to choose from in 14" sizes these days. Certain 15" sizes (205/50) are still well-supported, due to the plethora of pocket-rocket/ricer and vintage cars using them. But all of the real tire-industry efforts are in the 16" and up sizes these days.
Personally I think 15x6 Fuchs (5-lug) are the way to go on a 914 street car. Or, if you want to stay with a 4-lug setup but gain a little bit of width, Panasports are killer "vintage-looking" wheels. Of course the stock 15x5.5" 4-lug Fuchs are hard to beat for quality/looks/correctness too.