You can get the 86 plastics to fit but it requires quite a bit of work. The front fender is a direct bolt on.
I highly doubt the motor interchanges. At least easily. The drive sprocket is on the opposite side, and i believe the motor is larger on the 84's ( haven't had them side by side but the 85-86's seem to be smaller)
STEALTHDUNER
Trikes I Own
1983 ATC300R (Stealthy)
1986 ATC250R (ESR310)Twin))
1986 ATC250R (Stocker)
1985 ATC200X (Racer X)
1985 ATC350X (Thumper)
1984 ALT50
1986 TRX250R (ESR310 PV)
1988 TRX250R
1997 Yamaha Banshee
2004 TRX450R