Regarding a fully WW11 spec. replica radar tower at Swingate, had the demolished tower been the only original feature on site, maybe it would be more difficult to justify it. In the context of the whole site, however, the demolished tower is only one small but very prominent feature of a fairly comprehensive site. Replacement of the tower would therefore become a small but very important enhancement .
Regarding replicas in general, how much of HMS Victory is original. The same can be said for many of our "historical" survivors, be they buildings, ships, vehicles or whatever. I believe that historically speaking, our radar towers are just as important as the Victory or even the Tower of London. Better a rebuilt tower than no tower at all.