Hi Dan,
Historically, 20mtrs/sec for mean piston speed was taken as an upper limit, speeds now are up, on average , to the 26mtrs/sec area.
The Aprilia engine revving to its peak of 13,200rpm has a piston speed of 24mtrs/sec, your 58mm stroke Bantam engine at 10,750rpm is just shy of 21mtrs/sec, the GT186 with a stroke of 62mm would run all day long at 10,000rpm . If you can get 10,000+rpm then go for it, the quality of engine internals will probably be self determining. A 58 stroke with a 52mm piston on top will be safer than the same with a 66mm piston on top, weight will be a factor, and so on ! Flash readings of 10,000+rpm should not be too much of a problem, telemetry on the Aprilia revealed that one works rider on changing down too enthusiastically had the revs spin up to 17,000rpm, and he seemed to get through big end bearings more than any other rider!
Trevor