We have found with the genuine ART piston that it swells up more than the older design and if the bore is bang on spec and you rev it hard it traps the rings relatively quickly. However you say you have a nikasil bore, which I don't understand and I don't recommend honing it with a handtool.
Ok so I don't quite understend, I thought that XR2 Racing have nickasil as standard. Noone did that. Just have to polished (honig - if I do undestand that word ) cylinder.
I think you will find you don't have a nikasil motor. Visual inspection Nikasil would be a pale gold colour and retains it's crosshatching very well because it wears so well. Standar bore would be dull silver/grey much like a steel liner.
No because they are a caged unit, you cannot change the rollers as they are crimped in. As it is a bottom guided rod use the genuine parts I promise you it will work out cheaper than a new block.