Short answer: Not for motorcycle applications, and not really.
Radioisotopes are fantastic energy sources for long missions with no resupply possibility. See Mars rover. They have a very high total energy supplied per unit mass metric, which is perfect for a space mission which must operate for very long periods of time.
The radioisotope thermo-electric generator in the Curiosity rover weighs about 100 pounds, supplies 100 watts electricity and 2000 watts of heat. I don't think that weight includes shielding, which probably isn't a significant concern on Mars.
Using a liquid solution for electricity generation instead of a thermocouple could have some cool applications, and maybe it's more efficient or lighter or something. Without reading the paper, it doesn't really matter, because small amounts of onboard energy generation that require significant compromises are next to useless for a conventional electric vehicle.
An onboard power generator sufficiently large enough to power the bike at highway speeds (10 kW) would be too large, too expensive, and would waste the vast majority of its energy.
Imagine that you had 10 kW of magic solar cells that you could install on the bike. Fantastic, now you can ride it endlessly on the highway, at least when the sun is shining!
On a typical summer day these solar cells would supply 60-70 kWh of energy to the bike, or ~500 miles of range (< 200 miles/day in winter). Perfect for a long trip, but overkill for the typical 10-50 miles of travel most people do each day, and a waste of those magic solar cells.
The average person drives 12-15000 miles per year; that's 240-300 miles per week of travel on average. At 15k miles/year, the bike would drive 150k miles over a typical 10 year lifespan. Pretty good, right?
If connected to the grid instead, the 10 kW solar array would supply around 10 MWh per year, over 25 years they would supply about 220 MWh or enough energy for around 1.5 million miles.
The same calculations but even worse apply for an RTG. A magic bike-sized RTG that could supply 10 kW would power the bike for the same 150k miles over a 10 year lifetime; or if connected to the grid over a 10 year period @ 90% average capacity could supply ~800 MWh, or enough energy for 5+ million miles.
Coming up with a 10 kW no-fuel energy source of course is much harder than solving the V2G problem .. but put the renewable energy sources where they belong.
