Jeff,
You cannot invoke a flow action after the lead is deleted, because the lead's attributes like LeadID will no longer be available after the lead is deleted. This is why there is no trigger for "Lead is Deleted". So your best bet would be to invoke the Webhook before the lead is deleted from Marketo. You could store the result of the webhook call (ie, whether the lead was deleted in your custom CRM) in a custom field and delete the lead in Marketo only after confirming that the lead was deleted in the CRM. This will help you ensure that both systems are in sync
Raj