Dual Flush Valve Fittings

[cam777]

I'm in the process of trying to replace a leaking flush valve with a Flomasta Dual flush Fill valve kit. The problem I've got is trying to get the flush pipe connected into the base of the valve. The existing flush pipe is marginally too small, but might be OK with a suitable gasket but I'm not sure what size gasket I would need - my local plumber appears to have closed down. If you have any experience with this valve, appreciate if you can let me know what fittings are needed to correctly fit the flush pipe.
Thanks!
Cam

 

[WC1]

Might try

https://www.NoLinkingToThis/p/eurof...roiqQncYe2grPcDrv2hoCagMQAvD_BwE&gclsrc=aw.ds