Hi Andrew and
to the forum.
Those 18 and 16 tooth sprockets should fit, but they would not allow you to pedal comfortably at higher speeds as your legs would struggle to keep up with the low gearing.
Is there such a thing as a freewheel spacer?
The
fatbike version of the MP5 appears to have a very wide spacer threaded onto the standard hub casing, but it might be too wide for the standard axle.
I don't think that chain alignment will be an issue, as the single gear would not be any closer to the hub than the 1st gear on a conventional freewheel cluster:
If your frame does not allow the axle to move forwards and backwards to tension the chain, then you would still need to use a derailleur mechanism to keep the chain tension correct.
Would this work with the Magic Pie?
It should be possible to fit that fixed gear unit onto a Magic Pie, but why would you want to?
Without a freewheel device, the pedals would be very dangerous spinning round at high speed even with the 13 tooth sprocket.
What are you hoping to gain from all the extra work and costs involved in swapping everything over?
Alan