Losing a magazine is always annoying, and can be costly, as they are usually expensive to replace.
It is even more upsetting in your case, as you have a classic old rifle and it will be extremely difficult to replace the magazine.
Trying to find a replacement magazine for any class of rifle invariably presents problems, but even more so for a rare or classic item.
Your best course of action is to contact GMK Ltd (tel 01489 587500), which is the official importer of Sako rifles.
However, the Model 78 has been out of production for decades so I doubt that they will be able to find a spare magazine for you.
There is a possibility that the Finnish factory might have one lying around, but it is a pretty slim chance.
Alternatively, there are many small traders who attend the meetings at Bisley Camp, and who often have boxes filled with old sights, magazines, stocks and so on, so you might strike lucky there.
The rifle is, of course, useless without a magazine, but you might be able to part-exchange it for a newer model.
In fact, the Sako Quad, with its quick-change barrels, might be the answer — but buy one or two spare magazines this time!