And that means ...
it is not the original engine.
I suspect that a 7210 was originally a Colt Minor - with the 10 Series 4hp.
[i.e. Model 72 + 10 Series]
This would be consistent with the 7313 being the 13 Series
[i.e. Model 73 + 13 Series]
Colt Minors (or Colt 4hp) would be rare these days.
Many dealers simply preferred to sell the 5hp machine.
Both the 10 & 13 Series shared the same design philosophy:
smaller bore; longer stroke. With their cast iron flywheel,
both were torquey motors, and both loved by repairers.
The 10 Series ended production in the early 1980s. It was replaced
with the 11 series 4hp and the 4hp Colt became a Model 7221.
It's up to you how you proceed, but it might be nice to preserve
a 7210 in its original form. Here is a 10 series Briggs:-