Guys
Respect your advice and knowledge, HOWEVER........ the Bell 47 is not an EASA TC recognised aircraft. It is an Annexe 2 aircraft and thus down to the NAA (ie CAA in this case) to determine its airworthy compliance. Nothing to do with EASA
Aside from the technicalities and the O/H of the engine (would the CAA accept a top O/H only?) If so talk to Richard Von Eisenberg at Southend for either top or full, he's been doing it for years.
One would normally expect an export C of A to validate issue of a UK National Cof A, Oh and BTW you will need a UK BCAR (Sect L) endorsed licenced engineer to certify the work. Can't do it on a Part 66 licence
On this & related matters I spoke to CAA STN this past week. Some changes being promulgated to BCAR A8, particularly for component repair and O/H in a Part 145 approved shop, Either suffix -23/-24 or -25, all due to be published in April.
I would recommend speaking to A&A at Airfix House, John Nicholas is the man in charge I think. A little 'light on his feet' but he's OK if you tackle him in the right way.
If you need specifics PM me. (I do hold a CAA Sect L licence for the type) - VFR