A Part-FCL licence with single-engine piston aeroplane privileges is not deemed to be rendered valid for a Microlight aeroplane unless the holder of the licence has undergone differences training in accordance with the Air Navigation Order, Section 2 of Part B of Schedule 7, appropriate for a Microlight aeroplane class rating. A LAPL(A) does not have an SEP 'rating', but does have SEP privileges endorsed, therefore may be rendered valid for Microlight aeroplanes, subject to satisfying the ANO requirement.
The ORS4 No.1087 90 day rule exemption does NOT apply to the LAPL. ipso facto, whether you fly an SEP aeroplane or a microlight, the 90 day rule still applies.