Even the m45 2.75 18 is a tight fit. It is a few mm wider than the original tyre which is labelled as 2.75 18. So with the standard guard you won't have an option of going any bigger. And it has been posted elsewhere that you need to keep the guard as it is structural. Others have found that if they run the bike as pictured, with no guard, the forks move around! I guess you could get a fork brace made up to replace the front guard and make sure that has clearance for a bigger tyre. Others will be able to comment on how big a tyre the rim may be able to take safely.
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