The easiest way to fix this is spin up a trial or get access to another environment that has the buttons. Just copy everything exactly using the ribbon workbench and the buttons will work as if they had never been removed.
Here is the xml if you want to take that approach
<CustomAction Id="new.Mscrm.Form.opportunity.MarkAsWon.CustomAction" Location="Mscrm.Form.opportunity.MainTab.Actions.Controls._children" Sequence="3">
<CommandUIDefinition>
<Button Alt="$Resources:Ribbon.Form.opportunity.MainTab.Actions.MarkAsWon" Command="Mscrm.Form.opportunity.MarkAsWon" Id="Mscrm.Form.opportunity.MarkAsWon" Image32by32="/_imgs/SFA/MarkAsWon_32.png" Image16by16="/_imgs/SFA/MarkAsWon_16.png" LabelText="$Resources:Ribbon.Form.opportunity.MainTab.Actions.MarkAsWon" Sequence="3" TemplateAlias="o1" ToolTipTitle="$Resources:Mscrm_Form_opportunity_MainTab_Actions_MarkAsWon_ToolTipTitle" ToolTipDescription="$Resources:Mscrm_Form_opportunity_MainTab_Actions_MarkAsWon_ToolTipDescription" ModernImage="MarkAsWon" />
</CommandUIDefinition>
</CustomAction>
<CustomAction Id="new.Mscrm.Form.opportunity.MarkAsLost.CustomAction" Location="Mscrm.Form.opportunity.MainTab.Actions.Controls._children" Sequence="4">
<CommandUIDefinition>
<Button Alt="$Resources:Ribbon.Form.opportunity.MainTab.Actions.MarkAsLost" Command="Mscrm.Form.opportunity.MarkAsLost" Id="Mscrm.Form.opportunity.MarkAsLost" Image32by32="/_imgs/SFA/MarkAsLost_32.png" Image16by16="/_imgs/SFA/MarkAsLost_16.png" LabelText="$Resources:Ribbon.Form.opportunity.MainTab.Actions.MarkAsLost" Sequence="4" TemplateAlias="o1" ToolTipTitle="$Resources:Mscrm_Form_opportunity_MainTab_Actions_MarkAsLost_ToolTipTitle" ToolTipDescription="$Resources:Mscrm_Form_opportunity_MainTab_Actions_MarkAsLost_ToolTipDescription" ModernImage="MarkAsLost" />
</CommandUIDefinition>
</CustomAction>
</CommandDefinition>
<CommandDefinition Id="Mscrm.Form.opportunity.MarkAsWon">
<EnableRules>
<EnableRule Id="Mscrm.CanWritePrimary" />
<EnableRule Id="Mscrm.OpportunityIsOpen" />
<EnableRule Id="Mscrm.IsAvailableInMocaOffline" />
</EnableRules>
<DisplayRules>
<DisplayRule Id="Mscrm.CanWriteOpportunity" />
</DisplayRules>
<Actions>
<JavaScriptFunction FunctionName="Mscrm.OpportunityCommandActions.opportunityClose" Library="$webresource:Opportunity_main_system_library.js">
<BoolParameter Value="true" />
</JavaScriptFunction>
</Actions>
</CommandDefinition>
<CommandDefinition Id="Mscrm.Form.opportunity.MarkAsLost">
<EnableRules>
<EnableRule Id="Mscrm.CanWritePrimary" />
<EnableRule Id="Mscrm.OpportunityIsOpen" />
<EnableRule Id="Mscrm.IsAvailableInMocaOffline" />
</EnableRules>
<DisplayRules>
<DisplayRule Id="Mscrm.CanWriteOpportunity" />
</DisplayRules>
<Actions>
<JavaScriptFunction FunctionName="Mscrm.OpportunityCommandActions.opportunityClose" Library="$webresource:Opportunity_main_system_library.js">
<BoolParameter Value="false" />
</JavaScriptFunction>
</Actions>
</CommandDefinition>