Well you sound like you have youhave everything figured out for your 390 e==========================================
An Engine doesn't know what it's on, only People Do!
Engine Life(Hrs) has more to do with the Person who Builds it, the Person using it, and Person Maintaining it, using Good Fresh Octane Gas with Good Oils, and doing Good General Maintenace, Plugs, Timing, Oil Changes, etc!
Old Stock Cont. 084ci, 4 Stroke Motors, off Military 10kw Generators used on Kitplanes at 3600rpm made over 1000hrs, 20+ years ago. Today, you have better Oils, Metals, Technolgy, to use in these Motors. Simonini 2 Strokes have a 600hr TBO, Hirth 2 Strokes a 1000hr TBO, Rotax 2 strokes 300hr TBO, all using Higher Rpms. Rotax 912(80hp) 4 Stroke 1500+hr TBO used at 5500rpm continious.
Yes, I did say, "Continuous 4800-5000rpm on a Mini Air Boat or Snow Glider, Hence Endurance is the key Factor.
You have Set variables Max 5500rpm, a Max Safe CR for 93 Octane, Max 389cc, everything else is up for whats best to use!