Is it viable to use cottered cranks?
Give them a try. 

The main reasons they used to fail, if my boyhood memories are accurate, was incorrect installation.

When installed firmly they would last for years.

The important thing is proper support for the bike and pedal arm while installing the cotter pins. The pins must be hammered in firmly, without transmitting any of the hammer impact to the BB bearings. Destroying the bearings was common. After installing the pins firmly the nut on each should be tightened to hold them firm. 

If you have a source of the correct cotter pins then trying will not cost much. If you find them to be too much trouble then you can always such to plan B: a square taper BB.

The Spirit crank and chain rings were integrated, so you'd need new rings of you replace the BB.