I see no reason why it would not be possible for a competent machine shop in France to install a pair of seat inserts into your cylinder head.
The only points to watch out for are:
For best results, fit new guides before the seat inserts are fitted, as the valve seat insert machine will use a pilot fitted to the guide, and worn guides will effect accurate placement of the seat insert with respect to the valve guide bore.
The valve seat depth in the cylinder head, with the seat insert fitted the valve should be between 0.055" to 0.100" below the joint face. if you aim for as close to 0.055" then you will have ample material left on those seat inserts for future valve seat recutting.
The seat insert selected for the job will need to match the port size, to small on the i/d and it can restrict flow, to large on the o/d and it may weaken the cylinder head, leading to cracking.
After fitting the valve seat inserts, they should be blended into the ports wherever possible to remove any sharp corners, to improve flow.
When cutting the valve seat sealing face keep the seat width narrow, as the valve spring pressure on CS engine is not great.
Its quite possible that if you have 3-1 flywheels, and a 6-1 piston. Then the balance of the engine will be effected, this will depend upon the weight difference between the 3-1 piston and 6-1 piston. Have you also changed the connecting rod, as the 3-1 rod has a narrower small end bush.
This link gives a good description of the balancing process for a single cylinder engine:
http://modelenginenews.org/etw/etw_bal/p2.html
