[inebriate]

first, backup skin1/modules/product_options/customer_options.tpl

then replace

<select id="po{$v.classid}" name="{$poname}"{if $disable} disabled="disabled"{/if}{if $nojs ne 'Y'} onchange="javascript: check_options();"{/if}>
{foreach from=$v.options item=o}
	<option value="{$o.optionid}"{if $o.selected eq 'Y'} selected="selected"{/if}>{$o.option_name}{if $v.is_modifier eq 'Y' && $o.price_modifier ne 0} ({if $o.modifier_type ne '%'}{include file="currency.tpl" value=$o.price_modifier display_sign=1 plain_text_message=1}{else}{$o.price_modifier}%{/if}){/if}</option>
{/foreach}
</select>

with

{foreach from=$v.options item=o}
<input id="po{$v.classid}" type="radio" name="{$poname}" value="{$o.optionid}" />{$o.option_name}
<br />
{/foreach}

as for having a selection required before checkout, its a bit more involved and it may mess with the form validation since to check if there is something selected for radio buttons you need to loop through all of them