The Viking 21's are excellent. I had one last year- I had a tough time replacing one of the drive belts, as the local OSMG (I was in Portland, OR at the time) insisted that no belt would be available. I firmly reiterated that I dabble in old machines and had NO DESIRE to buy a new machine ( as he was obviously pushing me in that direction.) After he finally got the message, he shuffled away to the parts room and came back with two belts, one of which fit perfectly!
The low speed gear reduction on these machines are really nicely done. On the Model 21 the low speed gear reduction was mostly metal, although there was one gear that was probably nylon. But even that gear looked like it would last another 40 years. I now have a Husqvarna 1030, which is a later and simpler machine (1974-1975) and still very solid, but by that time the gears were all plastic. Still good, maybe only good for 10 or 20 more years!
I do regret having to sell that Model 21, but needed the money to buy other machines at the time. The young lady who bought it from me later sent me some great picture of her first project made with the machine - a tremendous dragon costume for her 4-year old nephew. It was impressive work (and once again made me want to learn how to sew!), and she couldn't have been happier with her "new" Model 21.