There is bound to be a mechanic or retired mechanic around Perth who has a set of Warren & Brown (a division of Repco in the old days) valve seat cutters - every corner garage used to have them. It is just a matter of choosing a diameter, seat angle and pilot diameter to match the new valves, from the wide assortment in the W&B set. Because your valve stems are oversize it may be necessary to turn up a pilot on a lathe, which should be a very small job. Then the actual seat cutting is a five minute job.
If the seats are severely undercut (i.e. are well below the level of the surrounding combustion chamber) this shrouds the valves, which reduces the engine's power output somewhat. A small amount of undercutting is not important for a low-performance lawnmower engine. However do not cut deeply because that would leave the valve seat inserts too thin for stability and durability.
