Regarding mortages:
As others have stated, it is possible to buy croftland that has a decrofted house site and/or a house already built on the decrofted site. This gives the banks the security they need. I know several who have purchased, via mortgages, like this.
Bare land crofts are a different matter.
It should be noted that even an owner occ croft-OOC- (where the croft is owned outright) can, "theoretically", still be influenced by the crofting commission. If the OOC is not being used for intended purposes the CC could force the owners to let the croft. therefore the owners would then become landlords and the leasee a tennant. The 34km (why km I'll never know, when did we shift to using km?!?) also applies to OOC.
In reality it is very difficult (the CC are veeeery sloooow) to act upon unused croft or absentee tennants/owners. One day, we can hope, that there will be more drive to ensure crofts are used for crofting.
Rgds
Sskye