Welcome!
The MR175 can be very hard on flywheel keys - my original one was nearly sheared in half. I've found decent complete motors on eBay for less than $200 - sometimes less than $50 - but right now (12/16/15) doesn't seem to be a good time for either motors or cranks over there. Just for what it's worth, my buddy is currently running an aftermarket Powerdynamo ignition on his MR175 with no key at all (!), and it hasn't slipped so far (both tapers are perfect, though).
The MR175 ignition can be a real horrorshow after 2K miles or so, although most of them work OK early on. The MT125 ignition fits the motor cases & ignition cover, but you'll have to adapt the cable to the MR harness. I'm using CR125M ignitions on both my 175's, which make them feel more like 250's down low.
Re the clutch - I'm not sure if by 'posts' you mean the threaded posts in the hub, or what I call the 'fingers' in the basket/primary gear that engage the clutch plates. The '75-'78 CR125M or '75-'76 MT125 clutch hubs (with smaller center hole) will work, but only the MR basket will fit unless you change out the basket and drive gear to the CR125M setup, which is what I've done in both my motors.
Ray
_________________ '74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric
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