The IRS has made it explicit that conversions of 2017 distributions can still be recharacterized.
As I interpret your question, your account is now worth $290,856.34 / $271,895.67 = 1.069735 times what is was worth immediately following the conversion and you made no other contributions to or distributions from the Roth IRA after the conversion contribution to the Roth IRA.
I think you meant to say that you want to recharacterize $89,127.85 - $83,346.85 = $5,781 of the original conversion (not $6,781). To recharacterize $5,781 back to the traditional IRA, shares worth $5,781 * 1.069735 = $6,184.14 need to be transferred back. If that is to all be in the form of Y company stock presently valued at $8.95/share, that would mean transferring back 690.9652 shares.
Of course the numbers change with the share prices on a daily basis, so, as Hal_Al said, the calculation really needs to be done by the custodian if you want an accurate recharacterization of $5,781. If you specify the number of shares yourself, you might get close but the actual amount recharacterized will be something more or less since Y company shares are not the only shares determining the Roth IRA balance.
It's rather irresponsible of the custodian not to do the calculation. It's a matter of tax law, not a tax question. Most IRA custodians do the calculation for you.
