Increasing the width also increases the overall diameter - a 155/80R13 tyre is 32mm bigger than a 135/80R13. You'd mess up the gearing (and the speedometer accuracy) if you were doing this on a car, but it isn't a car, so it isn't as important.
It comes down to whether there is sufficient clearance for the bigger tyres, really.
I wouldn't have thought it would be a problem, not least because on the official pressure chart on this page here it lists 155/80R13s as the smallest standard fitment and doesn't even mention 135/80s.
I reckon you'll be okay.
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