The answer to "is there a replacement aircraft for the DC3?", is Joe McBryan doesn't give a sh*t, he doesn't want to replace them. He hates jets and it has taken years and years for his son an friends to even convince him to look at turboprops. Even then, they go out and buy what are considered old equipment like the Electra.
Sounds like the same business model followed by Reeve Aleutian Airways' Bob Reeves, 25 years later. Reeve eventually went out of business in 2000, RIP.
The capital cost of new equipment is the main impediment to any frontier operator, who cannot get the consistent utilisation to amortize the investment required for 'modern' aircraft. Boom and bust resource based economies, ball-busting weather and not enough good people prepared to work and raise a family in such a challenging environment are just some of the distractions.
Finding a replacement for life expired, gravel kitted B732's is almost impossible today. I believe Boeing charges around $750k just to authorize the transfer of the gravel kit hardware and documentation from one old used 732 to another, only slightly less old aircraft! And, of course, the market is too small to justify the manufacturer certifying a new aircraft for gravel operations.
I truly hope Buffalo Joe thrives and survives for many more years, he deserves every success.