Whilst I take Steveb's point about it being different for different makes are they so different that any make wouldn't give a rough idea? I have done the same myself but found it difficult to get info on radiators without convector fins as not many seem to be sold now. I only needed single panel I haven't seen any data on double panel no convector fins (hence your post).
Can you still buy the double panel without fins? I haven't seen any. Another approach if you cannot get them is to ask was this radiator sufficient when at it's best? If not or borderline get double panel with convector (check whether the pipework will reach it). If it was a bit much then a single panel with convector fins.
The nearest size I found for your figures is the Stelrad 450x2200. The single panel single fin is 1690W at deltaT 50 and the double panel single fin is 2433W so yours was probably somewhere in between when new.