I have similar problem for a 15 years old .32 heli engine. You may try taking out glow plug and spray WD40. You may also spray to the con-rod if possible.
I also run-in a new HPI engine last week, for the first few pull, very very very hard and need very much much power to pull. I also worried of string broke at that time. But eventually, it was OK. Loose the glow plug will make it easier to turn.
The risk is quite high if hitting the piston by something.
