I know you said no yz250f motors but that would make a way faster trike then a 350x. The motor would cost twice as much is the bad part.
from looking at peoples Triz's over the years i dought it will work without major frame mods.
JASON LISKEY
1985 atc 250r ---- finsh line axle,.fmf exhaust and rictor privateer silencer, prm grab bar.
2008 KTM 525xc quad ----- HMF exhaust, HRE chromoly stem,
86 trx 250r --- with after market chromoly frame and everything else done to it
01 400ex --- with 86 trx 250r motor
87 250x + 93 300ex both 350x powered
2 = 86 atc 350x's
86 atc 250r
83 atc 185s --- from retired guy not ridden since 87
Bandito framed 110 with 85 motor and yellow us90 fenders + tank