AVB, thanks for the rundown on the hone type, and pic. As you'd gather, commercial workshops here don't find it worthwhile to resize Briggs bores! And 'backyarder' repairers generally don't shell out for the special tools required.
Chris86, the benchmark on cost is, that a 550 series 3.5hp sloper Briggs can be had for under $300 new, with 2 year warranty. And that gets you an OHV engine with cast iron sleeve bore. 'Chondas' are even cheaper, but it may be difficult to find one with the right shaft size for the SB clutch.
So it's just not worth putting a lot of coin into parts for the old Briggs, as I see it.
As a guide, the gasket set will cost $30-60 [quality aftermarket vs genuine]. That, plus seals, carby pump diaphragm and a ring set, will get you to around $100 or so - personally I wouldn't be inclined to go any further. Put it back together with those, set the points, do the valve clearances, and it will run for years. Oh, replace the spark plug too.
I'd agree with you that motorcycle, and even go-kart, mechanics are used to dealing with much higher-tech engines than this low-compression alloy bore side-valve! So there are things they just don't get, about what can and can't be done successfully on these.
