Engine vibration

Hi I'm about to buy a griff 500, but the engine is vibrating (like an oscillating feeling) about 2500rpm. Any ideas what it could be, I was thinking harmonic balancer or maybe flywheel in some way? I know there are issues with the crank pulley and the bolt etc, maybe this is a factor?

Cheers for the heads up. Never simple is it!?

im researching like a mad man!

best bet at the moment as the rest of the car is so good is to buy it and try and sort the imbalance short term, i plan to keep the car for a long time, so next year i will probably rebuild the bottom end with a non chocolate crank etc, the top end has already been overhauled and the nice bits like ECU, Cam and twin carbon plenum have been added.