Bob Would a standard crash helmet be sufficient? If so your local motorcycle store. Another type that might suffice would helmets used in the horsey world - local horse supplies or saddlery

Roll-over protection CAN usually be retrofitted. Where the machine is used on slopes then RoP is surely an absolute (in conjunction with a seat belt of course). Not entirely sure what risk you think head protection would mitigate. Motor cycle type helmet would not be suitable for grass-cutting operations, too limiting on operator field of vision. http://www.hse.gov.uk/pubns/misc175.pdf well worth a read.

Bob I would not expect any helmet to give adequate protection in a roll over situation. Roll over protection and personal restraint is the answer. Steve

Bob, the absence of a ready commercial solution to RoP for your machine does not absolve you of appropriate PUWER Reg 26 duty & action where hazard exists (see AIS27 reference above).