Watfordhorn from this form had an NC30 framed RG500. Went like stink and was what convinced me to buy a 4 cylinder 2stroke (YZR500 rep unfinished)

There is a guy who specialises building in them and doesn't live a million miles from London. His name escapes me, but a google search is bound to find him.

The trouble with an NSR frame is the top of rear shock mount is too high and getting those pipes over the top is difficult. The nc30 has a lower mount from recollection.