A "HT" Chinese in frame kit could very well make it, I've personally logged thousands of miles on them - the catches are the poor quality control & setup/dialing it in.
For an adventure like this yes, I'd have a spare motor kit - but I'd leave it w/friends and bring just some basic repair items. If I ran into serious trouble having your friends send you the complete replacement motor would be easy enough & fixing/replacing these things is simplicity itself.
The more problematic issue is the initial build, setting up the bike to be as simple & reliable as possible and logging enough miles to "prove" the motor, making sure you didn't end up w/a wonky one. In this I doubt any of the four stroke kits would be any different, even if the engine itself was completely "bullet proof" there's still the oddities involved in converting any bicycle with any
My recommendation would be get w/e engine suits you, your budget and your bike the best & run it a while to work the bugs out, once yer confident in your machine - then consider an epic journey