No, sorry - won't work. Kawasaki stopped casting power valves on the 86 and 89 KX125 years ago. They are obsolete aka no longer available.
I checked with a local gear manufacturing facility who said they could reproduce the same power valve, but the price was insane - $1200 for one.
You can verify fitmet by checking Partzilla's model reference tool. I'm including a third link for some footpegs which appear on multiple models so you can see how it works.
1986 KX125-E1 KX125 FOOTRESTS
1986 KX250-D2 KX250 FOOTRESTS
1986 KX500-B2 KX500 FOOTRESTS
1987 KX125-E2 KX125 FOOTRESTS
1987 KX250-E1 KX250 FOOTRESTS
1987 KX500-C1 KX500 FOOTRESTS
1986 KDX200-C1 KDX200 FOOTRESTS
1986 KX125-E1 KX125 FOOTRESTS
1986 KX250-D2 KX250 FOOTRESTS
1986 KX500-B2 KX500 FOOTRESTS
1987 KDX200-C2 KDX200 FOOTRESTS
1987 KX125-E2 KX125 FOOTRESTS
1987 KX250-E1 KX250 FOOTRESTS
1987 KX500-C1 KX500 FOOTRESTS
No not the bumpers.
Valerie - 1986 Paris Part 1 5-1 was released on: USA: 18 September 1989
Valerie - 1986 Paris Part 2 5-2 was released on: USA: 25 September 1989
Valerie - 1986 Paris Part 3 5-3 was released on: USA: 2 October 1989
Peter the Great - 1986 Part IV 1-4 was released on: USA: 5 February 1986 West Germany: 21 December 1986 Japan: 5 October 1989
Matlock - 1986 The Hunting Party Part 1 4-1 was released on: USA: 19 September 1989
Matlock - 1986 The Trial Part 2 5-19 was released on: USA: 26 February 1991
Head of the Class - 1986 The Bright Stuff Part 1 4-6 was released on: USA: 1 November 1989
Head of the Class - 1986 The Bright Stuff Part 2 4-7 was released on: USA: 8 November 1989
Amen - 1986 She's in the Army Now Part 1 4-9 was released on: USA: 25 November 1989
The Judge - 1986 II Taking Care of Daddy Part 1 4-94 was released on: USA: 1 September 1989
Perfect Strangers - 1986 Father Knows Best Part 1 5-7 was released on: USA: 10 November 1989