I am no expert but have been sniffing around a bit.
Apparently the clutch issue is no worries. You could take your current friction plate to a clutch joint and get them to put a new spline in the centre for whatever box you are using. If not then they should be able to do a custom one.
Will certainly need a new tailshaft because of the different length of box. No big deal there though either.
What puzzles me is the fitting of clutch slave, clutch fork, carrier bearing etc...
Castlemaine rod shop (run by Rod Hadfield I think) does a good bell housing for 4a to W5* and it comes with all parts needed. Dellows also do one but I haven't heard rave reviews. Or talk to AZA510 on here, he has made adapter kits to fit W5* onto the back of a 4a bellhousing.
Should give you some people to talk to to get the ball rolling.
Hen
On the other hand I haven't killed a T50 in 6 months and I do stupid things constantly. Maybe just lucky.