when my 1.3 box began to slip out of 5th gear after a hard trackday (I could even hold it in place when applying any torque), I dismantled it all and found it was caused by worn mashed up teeth on the selector ring
(worn teeth: bottom vs newer teeth: top)
View attachment 52978
so here's the worn teeth on one side of the ring that engages 5th gear
View attachment 52979
I found that the selector ring is made symmetrical so there's another set of fresh teeth on the other side that's never used.
View attachment 52980
you could flip the selector ring around for a fresh set of teeth which may stop it slipping out of 5th (assuming the engaging teeth on the input shaft are also in good condition).
https://www.micra.org.uk/threads/pollymobiles-rebuild.35251/page-119#post-706020