Andy
If Mr J has all the parts you require hidden away then problem solved but just in case.
In answer to your original question I believe the 11F crank and rods will fit the R cases. In fact I am sure Keith from the CBF club has a 900 crank and rods in one of his 1100 F engines, so that too may be an option if all else fails.:confused:
To fit the 900 parts I would think you will also need the 900 primary chain, primary chain tensioner and primary drive but I’m sure Keith has done it and it will work. Drop Keith a mail he’ll be happy to help. When I spoke to Keith about this he said apparently it’s a common modification on race 11F engines because the smaller primary chain wheel on the 900 crank means the 900 crank is less likely to bend.