RE: EF Yak 54 68" engine?
If you can find one (EF54) grab it now. YS110 is the power plant--a lot of others will work well also. Just about anything that can spin at least a 16 x6 apc 9200+rpm and weigh less than 40oz with muffler will work good.
My YS 110 weighs 27.5oz and spins the 16 x6 apc 9,500 on CP 30%