Jeff with 400 square inches the 70 is the best engine for flying and with some nitro this plane will do 145-150 with the 70 on 50% nitro and a 12x12 prop. The YS 90 is an awesome engine but the weight is about 12 ounces more than the 70. Tdstaf on here designs these kits as he draws the plans himself and has short kits cut out for you. If you have been some kits from scratch or from a kit then this is up your alley. I recommend you get in touch with him. He has some kits in the large 515 square inch size that fit that 91 very well and latter if want even more can opt for a YS115WS. His 515 sq" planes with YS 115WS, 50% nitro and a 13.5x13.5 APC prop will do a min of 160. My Tsunami of his with a 14x14 and 60% nitro did a two way average of 184MPH.