Any FX, FR or FS engine will fit provided they have the same crank diameter as they come in both 1" & 1 &1/8"
The only difference in the FX from the FR & FS is the carb & the blower housing.
Thus your cheapest repair will be to buy an FS 651 and swap over the blower housing
Forgot about the carbs
The 3 engines are built on the same short block so they will all interchange
as is the 691 ( you check by the bore & stroke ).
So the cheapest has a small alternator, single barrel carb ,plastic blower housing and internal filter ( which is not particularly good )
Net price jump gets you a bigger alternator & a 2 barrel carb
Next step gives you a metal blower housing & external Donaldson type air filter
Small Engine Suppliers
Small Engine Warehouse
Should both have some FS or FR 651's
If not Repower definately will
as a rule of thumb you can ignore Hp it is meaningless when it comes to repowering an engine.
Engines are built in crankcases and most suppliers will start with a bore & stroke, the increase the bore
Then increase the stroke
Then increase the bore
Then increase the stroke
And on and on
Usually engines of the same stroke will interchange
Engines with a longer stroke tend to be longer & wider as they have bigger crankshafts
So when looking for a replacement you look at the capacity of the engine not the Hp
Hp variations are done by changing the carb main jet and governor springs if at all .
So any single capacity engine will usually come in 3 Hp ratings