4.5% may be manageable. It means you need a 1100W heat source to generate 50W of power. That’s about 3800 BTU/h. That’s much lower than what a portable camping stove can output. Propane has an energy capacity of about 24,000 BTU/liter, so in theory you only use about 0.2 liter per hour. Of course that doesn’t take into account the heat lost (without going into the generator), but you can probably get it down to about 0.4 liter/hour. That’s about 10 hours per gallon.
Then again, a 12-volt 40-Ah SLA (sealed lead acid) battery would last about 10 hours, and cost less than $200, and available off-the-shelf. They’re a bit heavy, but probably not much worse than a homemade generator setup. This one has a 40Ah capacity, costs $150 and weighs 38.5 lb.
For RC engines, you might want to explore four-stroke motors that are used more frequently for airplanes.
They are similar in size (the smaller airplane ones are similar in size to the big surface engines ), have lower HP but higher torque numbers… but run quieter and more efficiently… and they have a greater number of muffler/silencer options available. They produce their power much lower in the RPM range and have a lower red line and longer operating life. Less maintenance, too!
If you think about the nitro R/C engines that power cars/trucks and even boats, they are messy/fussy and work like an all out Indy race engine, which means they have a small sweet/fussy spot for tuning and require a person to care and feed them 24/7. They require diligence on a scale that can wear out even the most dedicated engine aficionado.