This is the optimal (but very demanding) bikepacking route from Reisadalen to Kilpisjärvi, based on five different scoutings.
The terrain up there is rough, and the marked Nordkalottleden footpath is mostly unrideable, especially from Kobmajoki to Meekonjärvi and from Guonjarvaggi to Saarijärvi. Fortunately the Sami people move around the area with their ATVs. This route follows the ATV tracks when they are better than the footpath.
There are two traverses without any path: From Kobmajoki hut to the beginning of the ATV track (1,3 km) and from Guonjarvarri saddle to the faint ATV track near the reindeer fence (2,2 km). They are still worth it. You really don't want to use the Nordkalottleden there.
There are no other major obstacles on this route. The area near Pitsusjärvi is unrideable, but there footpath is less demanding to push than the very rough ATV track over Vuopmegasvarri. The water crossings are easy, except in early June.
This route is much, much better from Reisadalen to Kilpisjärvi than the other way, because you gain most of the elevation on a decent gravel road.
Alternatively you can start from Birtavarre, take the gravel road up to Halti/Guolasjavri, continue through Biedjovaggi on an easy ATV track, and join the route in Coalbevaggi.
The best bike for this demanding, rough route is a full suspension 29er trail bike. You will need the suspension. This is no place for rigid bikes.