John Doe wrote:

Question is whether those are field replaceable/repairable,

It depends on the exact nature of the problem, but I think it's
unlikely. Anything more than the most superficial repair work would mean
dealing with hardware that has been contaminated with hydrazine and
N2O4, both very nasty chemicals. Better to leave the problem unsolved
than to try fixing it if either of those are involved.

and if Russia has any plans to provide a new backup solution since
they knew from the outset that those engines wouldn't last for the
whole station lifetime.

The backup solution is Progress. Just remember that Mir was in the same
situation for almost all of its life[1].


[1] Kvant-1 was in the way of the base block's engines.

--Chris