I've replaced just one Franklin smoke unit with the kit sold by Portlines and littlesteamers. I did use Stay-Brite solder and have had no melting issues. To be honest, I have not run the repaired Franklin engine for extended periods, so I can't comment on any long-term problems. So far it's been problem-free, however.
I agree that the rating on the resistor seems a bit low compared to the original (I think the one in my kit actually measured about 19.5 OHM). I see that Carl Tuveson sometimes swaps smoke resistors with the Lionel 27 ohm unit and still has good results. The Lionel 27 OHM unit is part #600-8141-055. ( Just for grins, I searched on eBay for the Lionel 27 OHM unit and found a source for a pack of ten for $10.99 with free shipping.)
As an aside, I checked the Lionel parts service and purchased a couple of complete Docksider smoke units for something like $13.50 each (it looks like the price has increased to $18.50). They are 18 OHM but a much nicer design than Gilbert's. I test fitted one to a Franklin and it looked like it would work. I think the only mod would be matching the mounting screw as it seems to be tapped slightly different. The 18 OHM Lionel resistor could be exchanged for the 27 OHM pretty easily if desired. I think the Docksider (and the Northern, too, I think...) us a regulator to control voltage to the 18 OHM smoke units, but I don't think that would be required with the 27 OHM.
I've not yet tried the conversion of the Docksider unit to the Franklin, but I don't see any reason that it couldn't be accomplished. If anyone has given it t try I'd be interested in the results.