The short answer is: I would continued to use that crankshaft.
The long answer is: I have not seen crankshaft wear limits for any trials bike. So we are forced to infer service limits from similar 2T engines.
A trials crankshaft has an easier life than one in an MX bike or roadracer. The average piston speed is fairly low, but the piston is also relatively heavy.
If you feel compelled to rebuild the crank, I probably would not replace just the big end bearing. Typically the rod, big end bearing and pin are all replaced at the same time.
Like their gearbox, OSSA did not sell any crankshaft parts separately. As a point of reference, the complete OSSA crankshaft (P/N 3000020211) had an MSRP of $725 USD back in 2013.
My best guess for something that would fit is GasGas TXT 250/280/300. They sell a connecting rod set (rod, pin, big end and small end bearings) as part number MT280212100. It has an MSRP of $266 USD today.
GasGas also offers the big end bearing alone as MT32050GG-AIZ-1. MSRP of $35 USD.
I have been very curious about the OSSA's connecting rod center-to-center length. Could you make that measurement for me?